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The chemical composition of the leaves of hardwood trees. The structure of hardwood

Wood

Wood (BESBE)

Wood(bot.). - In everyday life and technology, wood is called the inner part of the tree, lying under the bark. In botany under the name D., or xylem, refers to a tissue or collection of tissues formed from procambia or cambium(see this word and article. Woody plants); she is one of constituent parts vascular fibrous bundle and is usually opposed to another component of the bundle, originating from the same procambium or cambium - bast, or phloem. During the formation of vascular-fibrous bundles from procambium, 2 cases are observed: either all procambial cells turn into elements of D. and bast - so-called. closed bundles (higher spore, monocotyledonous and some dicotyledonous plants), or on the border between D. and the bast there remains a layer of active tissue - the cambium and bundles are obtained open(dicotyledons and gymnosperms). In the first case, the number of D. remains constant, and the plant is unable to thicken; in the second, thanks to the activity of the cambium, the number of D. increases every year, and the stem of the plant gradually thickens. In our tree species, D. lies closer to the center (axis) of the tree, and the bast is closer to the circle (periphery). In some other plants, a different mutual arrangement of D. and bast is observed (see. Vascular-fibrous bundles). D.'s composition includes already dead cellular elements with stiffened, mostly thick shells; the bast, on the contrary, is made up of living elements, with living protoplasm, cell sap, and a thin non-wooden shell. Although in the bast there are dead, thick-walled and stiff elements, and in D., on the contrary, they are alive, but from this, however, general rule does not change significantly. Both parts of the vascular-fibrous bundle also differ from each other in physiological function: along D. it rises up from the soil to the leaves, so-called. raw juice, that is, water with substances dissolved in it, but the educational one descends along the bast, otherwise plastic, juice (see. Juices in the plant). The phenomena of lignification of cells. shells are due to the impregnation of the cellulose shell with special substances, usually combined under the general name. lignin. The presence of lignin and, at the same time, the lignification of the shell is easily recognized by means of certain reactions. Due to lignification, plant shells become stronger, firmer and more resilient; however, with a slight permeability to water, they lose their ability to absorb water and swell.

Wood is composed of several elementary organs, otherwise histological elements. Following Sanio, there are 3 main groups, or systems, of elements in the D. of dicotyledonous and gymnospermous plants: the system parenchymal, luboidal and vascular. Each system has 2 types of elements, and in total there are 6 types of histological elements, and even cells of the core rays are attached as the 7th (see. Woody plants).

Wood

I. parenchymal system. It consists of 2 elements: woody(or wood) parenchyma and so on. replacement fibres. During the formation of cells of the woody parenchyma from the cambium, the cambial fibers are separated by horizontal partitions, so that a vertical row of cells is obtained from each fiber; while the end cells retain the pointed shape of the ends of the cambial fiber (see Table. Fig. 1 e- isolated by maceration of beech wood parenchyma cells; rice. 2 R- Wood parenchyma cells Ailanthus; tangential (see below) incision D.). Wood parenchyma cells are characterized by relatively thin walls; the latter are always without spiral thickening, but are provided with simple round closed pores. Reserve substances accumulate inside the cells in winter, mainly starch; but sometimes chlorophyll, tannins and crystals of oxalic calcium salt are also found in them. In addition, the wood parenchyma probably plays a role in the movement of water. As an integral element of D., it is very common; it, however, is very small in many conifers and completely absent, according to Sanio, in yew ( taxus baccata). The second element of the parenchymal system is replacement fibers (Ersatzfasern) - in some cases replace a missing woody parenchyma (hence the name); in others, they are found together with elements of the latter. In structure and function, they are similar to the cells of the woody parenchyma, but are formed from cambial fibers directly, that is, without prior separation of the latter by transverse partitions.

The article reproduced material from the Big Encyclopedic Dictionary of Brockhaus and Efron.

Wood (TSB)

Wood, xylem (from the Greek xýlon - tree), a complex tissue of woody and herbaceous plants that conducts water and mineral salts dissolved in it; part of the conducting bundle, formed from the procambium (primary D.) or cambium (secondary D.). It makes up the bulk of the trunk, roots and branches of woody plants.

Physiological and anatomical features of wood

Rice. 1. The main parts of the trunk and its main sections: 1 - transverse; 2 - radial; 3 - tangential.

The shape and size of the cells that make up the wood are different and depend on their functions. D. contains conductive, mechanical and storage elements. D.'s structure is typical for genera, and sometimes for species of woody plants. When studying D. and its properties, 3 main cuts are used, and for microscopic examination, cuts are used: transverse, tangential (tangential) and radial ( rice. one ). As the trees grow, the inner, oldest D. of the trunk dies off. The conductive elements of D. are gradually clogged: the vessels - the so-called tills, tracheids - the tori of their bordered pores. The conductive and storage systems cease to function, the content of water, starch, and partly fats in D. decreases, the amount of resins and tannins increases. In heartwood species (pine, larch, oak), the central part of the tree differs in color and is called the core, while the peripheral zone is called sapwood. In ripe tree species (spruce, linden), the peripheral part differs from the central one in lower humidity (such a tree is called ripe). In sapwood (maple, birch), the central part is no different from the peripheral. Sometimes, in sapwood and ripe wood species, the central part of the trunk turns darker (mainly under the influence of fungi) and a so-called false core is formed.

Rice. 2. Types of cells that make up wood: a - wood parenchyma; b - tracheids; in - segments of blood vessels (trachea); d - libriform fibers; e - cells of a heterogeneous core ray of a coniferous tree; f - cells of a heterogeneous heart-shaped ray of a deciduous tree.

In the wood of most dicotyledonous and all coniferous plants, growth rings, or growth rings, and radial, or core, rays can be distinguished. Within one ring of growth, early (spring) and late (summer) zones are distinguished, often called early and late D, respectively. Nutrients move along radial rays to their places of deposition. The dimensions and ratio of the elements that make up the vine vary depending on the growing conditions and the position of the vine in the stem. Under unfavorable conditions (excessive moisture, lack of water in the soil, strong shading, insects eating leaves), narrow layers of growth are formed. D. dicotyledonous plants are composed of the following types of cells: vascular segments (trachea), tracheids, mechanical fibers (libriform), woody parenchyma, and a number of other elements - transitional forms between them ( rice. 2 ).

Rice. Fig. 3. Scheme of the arrangement of wood vessels on the cross section of the annual ring: 1 - maple (scattered vascular); 2 - elm (annular).

Combinations in the size and arrangement of D.'s elements (for example, the diameter of vessels in different breeds varies from 0.0015 mm in boxwood and aralia up to 0.5 mm oak) create a variety of its structure ( rice. 3 ): diffuse vascular - throughout the growth ring, vessels of almost equal diameter, their number in the early and late zones is almost the same (birch, maple); annular vascular - the diameter of the vessels in the early zone of the ring is much larger than in the late one (oak, elm, maclura). Vessels can be located singly (oak) or in groups (ash, birch, aspen), in this case forming bordered pores at the points of contact. Tracheids in this case lose their water-conducting function during evolution and are replaced by libriform fibers (D. ash, for example, consists of vessels, wood and ray parenchyma, and libriform fibers).

Rice. 4. Plots of slices of pine wood: 1 - transverse; 2 - radial; 3 - tangential;
a - the boundary of the annual ring; b - late wood; c - early wood: d - a new row of wedged tracheids; e - heterogeneous core ray, consisting of ray tracheids (f) with small bordered pores and parenchymal cells with large fenestrated pores (g); h - resin passage (epithelial cells lining it are clearly visible); and - parenchyma cells surrounding the resin passage; to - bordered pores; l - core beam with a horizontal resin passage.

Wood also differs in the nature of the connection of the segments of the vessels, the form of perforation (simple, ladder, etc.), its location, the shape of the segment, the height and width of the medullary ray, and the shape of its cells. D. gymnosperms, including conifers, consists only of tracheids (vessels are absent), a small amount of wood parenchyma and medullary rays. In some genera (cypress, juniper), the core rays (homogeneous) consist of identical parenchymal cells; others (pine, spruce, larch) also have ray tracheids in heterogeneous rays, passing along the ray ( rice. 4 ). The structure of the beam, the shape of the cells, the number and size of their pores are important in determining the type of wood. Some genera (pine, spruce, Douglas fir, and larch) have resin canals in D..

The chemical composition of wood

Absolutely dry wood of all species contains on average (in%): 49.5 carbon; 6.3 hydrogen; 44.1 oxygen; 0.1 nitrogen. In D., the cell membranes account for about 95% of the mass. The main components of the shells are cellulose (43-56%) and lignin (19-30%), the rest: hemicelluloses, pectins, minerals (mainly calcium salts), a small amount of fats, essential oils, alkaloids, glycosides, etc. P. All D.'s cells are characterized by lignification - impregnation of the shells with lignin. There are more than 70 reactions to lignification (for example, phloroglucinol with concentrated hydrochloric acid gives a raspberry color). D. of some trees contains tannins (quebracho), dyes (logwood, sandalwood), balsams, resins, camphor, etc.

O. N. Chistyakova.

Physical properties of wood

The physical properties of wood are characterized by its appearance (color, luster, texture), density, humidity, hygroscopicity, heat capacity, etc. As a material, it is used in its natural form (timber, lumber), as well as after special physical and chemical processing (see. wood materials). An important decorative property and diagnostic feature is the color of D., whose characteristics vary widely (color tone 578-585 nm, color purity 30-60%, lightness 20-70%). Glitter is observed in D. of some hardwoods, especially on a radial section. Texture - D.'s drawing, formed when anatomical elements are cut, is especially effective in hardwoods.

D. contains free (in cell cavities) and bound (in cell membranes) moisture. moisture content of wood.

where W- humidity in %, m is the initial mass of the sample, m0 is the mass of the sample in a completely dry state. The limit of hygroscopicity (the saturation point of the fiber) is the state in which the fiber contains the maximum amount of bound (hygroscopic) moisture, and there is no free moisture. Humidity corresponding to the limit of hygroscopicity W pg at t 20°C, averages 30%.

Rice. 5. Dependence of the equilibrium moisture content of wood W p on humidity j and temperature t air.

Most wood properties are affected by changes in the bound moisture content. With a sufficiently long exposure, D. acquires an equilibrium moisture content. W p , which depends on humidity j and temperature t ambient air ( rice. 5 ). A decrease in the content of bound moisture causes a reduction in the linear dimensions and volume of wood - shrinkage. Shrinkage

where w- shrinkage in %, a pg - size (volume) of the sample at the limit of hygroscopicity, aw- the size (volume) of the sample at a given humidity W in the range 0- W pg. Complete (when removing all bound moisture) shrinkage in the tangential direction for all breeds 6-10%, in the radial direction 3-5%, along the fibers 0.1-0.3%; total volumetric shrinkage 12-15%.

With an increase in the content of bound moisture, as well as with the absorption of other liquids, swelling occurs - a phenomenon that is the opposite of shrinkage. Due to the difference in the values of radial and tangential shrinkage during drying (or moistening), transverse warping of lumber and blanks is observed. Longitudinal warping is most noticeable in sawn timber with defects in the structure of sawn timber. In the process of drying sawn timber, due to uneven removal of moisture and anisotropy of shrinkage, internal stresses arise, leading to cracking of sawn timber and round timber. After chamber drying, due to residual stresses in the diamond, during machining, a change in the specified dimensions and shape of the parts occurs. D. is permeable to liquids and gases, especially hardwood along the sapwood and along the fibers.

The density of the wood substance is the same for all species (because their chemical composition) and about 1.5 times the density of water. Due to the presence of cavities, the density of D. is less and varies significantly depending on the breed, growth conditions, and the position of the D. sample in the trunk. Density D. at a given humidity

where mw and v w- mass and volume of the sample at a given humidity W. With increase in humidity D.'s density increases. Often, for calculations, an indicator that does not depend on humidity is used - conditional density:

L = l nom × k r and kx are given in Tables 1 and 2. Temperature deformations of D. are much less than shrinkage and swelling and are usually not taken into account in calculations.

Some electrical and acoustic properties of D. are shown in Table 3. D. conifers with low density (spruce) has high resonant properties and is widely used in the music industry.

Table 1. - Coefficient kx

Table 2. - Coefficient To r

Mechanical properties of wood

The mechanical properties of wood are highest under the action of loads along the fibers; in the plane across the fibers, they decrease sharply. Table 4 shows the average properties of D. of some breeds with W= 12%. With increasing humidity up to W pg indicators decrease by 1.5-2 times. The modulus of elasticity along the fibers is 10-15 Gn/m 2(100-150 thousand people) kgf / cm 2), and across 20-25 times less. The coefficient of transverse deformation for different rocks and structural directions is in the range from 0.02 to 0.8.

D.'s ability to deform under load over time, which characterizes its rheological properties, increases sharply with increasing humidity and temperature. Strength under prolonged loads decreases. For example, the limit of long-term resistance in bending is 0.6-0.65 of the ultimate strength in standard tests for static bending. Under repeated loading, D. fatigue is observed, the endurance limit in bending is on average 0.2 of the static tensile strength.

D.'s tests for the purpose of definition of indicators of physical and mechanical and technological properties are carried out on small pure (without defects) samples. Tests are subjected to a series of samples, and the results of the experiments are processed by methods of variation statistics. All indicators lead to a single humidity - 12%. For most test methods, standards have been developed that establish the shape and dimensions of D. samples, the procedure for experiments, and methods for calculating indicators of its properties. D. is characterized by a strong variability of properties, therefore, when using D. as a structural material, it is especially important to use non-destructive methods for piece-by-piece testing of the strength of lumber, based, for example, on the relationship between the strength of D. and some of its physical properties. Wood defects (knots, rot, fiber inclination, list, etc.) affect the properties of timber.

When evaluating the properties of wood as a structural and ornamental material, one takes into account its ability to hold metal fasteners (nails, screws), wear resistance, and the ability to bend certain hardwoods.

D. has a high quality coefficient (the ratio of tensile strength to density), resists shock and vibration loads well, is easy to process and allows the manufacture of parts of complex configuration, is reliably connected in products and structures with the help of glue, and has high decorative properties. However, along with its positive properties, natural wood has a number of disadvantages: the dimensions and shape of parts change with fluctuations in humidity. Under unfavorable conditions of storage and operation (high humidity D., moderately heat air, contact with moist soil, moisture condensation on structural elements, etc.) D. rots. Rotting is the process of destruction of D. as a result of the vital activity of the fungi that settle on it. To protect against decay, D. is impregnated with antiseptics (see Antiseptics). D. can also be damaged by insects, for protection against which insecticides are used. In view of the relatively low fire resistance of wood, if necessary, it is impregnated with flame retardants.

Economic importance of wood

As a structural material, wood is widely used in construction (wooden structures, carpentry parts), and on railroads. transport and communication lines [sleepers, power line supports (power lines)], in the mining industry (support), in machine and shipbuilding, in the production of furniture, musical instruments, sports equipment; as a raw material in the pulp and paper industry and for other types of chemical processing (for example, hydrolysis, dry distillation), and also as a fuel. About the preparation of D., see Art. Logging.

Table 3. - Electrical and acoustic properties of wood

Indicators	Breed	Along the fibers	across the fibers
Indicators	Breed	Along the fibers	radial direction	tangential direction
Specific volume electrical resistance at W=8%, 10 8 ohm m	Larch	3,8	19	14,5
Specific volume electrical resistance at W=8%, 10 8 ohm m	Birch	4,2	86	-
Breakdown voltage at W= 8-9%, sq/cm	Beech	14	41,5	52
Breakdown voltage at W= 8-9%, sq/cm	Birch	15	59,8	-
The dielectric constant at W=0 and frequency 1000 Hz	Spruce	3,06	1,91	1,98
The dielectric constant at W=0 and frequency 1000 Hz	Beech	3,18	2,40	2,20
Loss tangent	Spruce	0,0625	0,0310	0,0345
Loss tangent	Beech	0,0585	0,0319	0,0298
sound propagation speed, m/sec	Pine	5030	1450	850
sound propagation speed, m/sec	Oak	4175	1665	1400

Table 4. Density and mechanical properties small clean (without defects) samples of wood at a moisture content of 12%

Indicators	Breed
Indicators	Larch	Pine	Spruce	Oak	Birch	Aspen
Density, kg / m 3	660	500	445	690	630	495
Tensile strength along the fibers, MN/m 2(kgf / cm 2): under compression	64,5 (645)	48,5 (485)	44,5 (445)	57,5 (575)	55,0(550)	42,5 (425)
in static bend	111,5 (1115)	86,0 (860)	79,5 (795)	107,5 (1075)	109,5(1095)	78,0 (780)
in tension	125,0 (1250)	103,5(1035)	103,0(1030)		168,0(1680)	125,5(1255)
chipping radial	9,9 (99)	7,5 (75)	6,9 (69)	10,2(102)	9,3 (93)	6,3 (63)
tangential	9,4 (94)	7,3 (73)	6,8 (68)	12,2 (122)	11,2 (112)	8.6 (86)
impact strength, kJ / m 2(kgf m / cm 2)	52 (0,53)	41 (0,42)	39 (0,40)	77 (0,78)	93 (0,95)	84 (0,86)
Hardness, MN/m 2(kgf / cm 2): end ..........…....	43,5 (435)	28,0 (285)	26,0 (260)	67,5 (675)	46,5 (465)	26,5 (265)
lateral......……......	29,0 (290)	24,0 (245)	18,0 (180)	52,5 (525)	35,0 (350)	20,0 (200)

Literature

Vanin S. I., Wood science, 3rd ed., M.-L., 1949;
Yatsenko-Khmelevsky A. A., Fundamentals and methods of anatomical studies of wood, M.-L., 1954;
Moskaleva V. E., The structure of wood and its change under physical and mechanical influences, M., 1957;
Vikhrov V. E., Diagnostic signs of wood of the most important forestry and forestry species of the USSR, M., 1959;
Nikitin N. I., Chemistry of wood and cellulose, M.-L., 1962;
Wood. Indicators of physical and mechanical properties, M., 1962;
Ugolev B. N., Testing of wood and wood materials, M., 1965;
Perelygin L. M., Wood science, 2nd ed., M., 1969;
Leontiev N. L., Wood testing technique, M., 1970;
Ugolev B. N., Deformability of wood and stress during drying, M., 1971.

B. N. Ugolev.

This article or section uses text

As in conifers, the core of hardwoods is formed by rather large parenchymal cells, among which sometimes there are small thick-walled cells located singly or in small groups and filled with brown contents; in birch, oak and ash, the core cells can remain alive up to 20 years of age.

Hardwood is built more complex and consists of a larger number of different elements, and in the cross section their radial arrangement is found only in the core rays. The strong development of individual elements, especially vessels, displaces neighboring cells, as a result of which hardwood does not have the correct structure that is characteristic of coniferous wood. The structure of hardwood includes conducting elements - vessels and tracheids, mechanical elements - libriform fibers and storage elements - parenchymal cells. Between these main types of elements there are transitional (intermediate) forms; this further complicates the structure of hardwood. On fig. 20 and 21 show diagrams of the microscopic structure of oak wood (annular vascular species) and birch (scattered vascular species).

Vessels - typical water-carrying elements of only hardwoods - are long thin-walled tubes formed from a long vertical row of short cells, called vessel segments, by dissolving the partitions between them. If at the same time one large rounded hole is formed in the septum, such perforation is called simple. If, after dissolution, a number of stripes remain in the septum, between which slot-like holes are located, then such a perforation is called stair perforation (Fig. 22). In many species there is one type of perforation in vessels, for example: oak has only simple ones, and birch has only stair perforations. Some breeds have both, but in this case, any one type of perforation predominates.

Rice. 20. Scheme of the microscopic structure of oak wood: 1 - annual layer; 2 - vessels; 3 - large vessel of the early zone; 4 - narrow vessel of the late zone; 5 wide core beam; 6 - narrow core beam; 7 - libriform.

After the connection of the cells that form the vessel, the protoplasm and nucleus die off and the vessels turn into dead capillary tubes filled with water. In large vessels, the diameter of the segments is large, while their length is often less than the diameter; partitions between segments are perpendicular to the length of the vessel, perforations are simple. In small vessels, the diameter of the segments is small, and their length is several times greater than the transverse dimensions; the septa between the segments are strongly inclined and, in many breeds, are equipped with stair perforations.

Rice. 21. Scheme of the microscopic structure of birch wood: 1 - annual layer; 2- vessels; 3- core rays; 4 - libriform.

Thus, the shape of the segments of the vessels can be different - from spindle-shaped in small vessels to cylindrical or barrel-shaped in large vessels; their length in early wood of annular hardwood species (large vessels) is from 0.23 to 0.39 mm, and in late wood (small vessels) from 0.27 to 0.58 mm. Side walls vessels of different breeds are distinguished by a variety of thickenings, which arise mostly by deposition of secondary layers on the primary shell, which in unthickened places remains cellulose and serves to pass water into neighboring elements; thickened places usually become woody, as they are intended to give strength to the wall of a vessel subject to pressure from neighboring elements.

Rice. Fig. 22. Details of the vessel structure: a - vessel segment with scalariform perforation; b - two vessel segments with simple perforation; c - spiral vessel; d - types of bordered pores on vessel walls; e - vessel with tills; 1 - rounded pores (birch); 2- diamond-shaped pores (maple); 3- multifaceted pores (elm); 4 - vessel wall; 5 - tills.

The thickening of the walls of the vessels is divided into annular, spiral and mesh (see Fig. 22). The least thickened are the annular vessels. Their thickenings are in the form of rings located at a noticeable distance from each other; such vessels are found only in primary wood. The wall of vessels with spiral thickenings is stronger strengthened. In mesh vessels, the wall is thickened almost entirely so that only pores remain, visible as frequent dots on the lateral surface of the vessel. In the wood of most hardwood species, mesh vessels are found, and in some species, for example, linden, maple, spiral vessels.

There are bordered pores at the points of contact of the walls with the neighboring vessel different shapes, which differ from the bordered pores of conifers in their smaller size and the absence of a torus. In places where the wall adjoins the parenchymal cells, the vessels have semi-bordered pores (bordering only from the side of the vessel). In places of contact with the cells of the medullary ray, there are rectangular areas on the walls of the vessel, on which oval or rounded pores with a very narrow border are closely located. In places of contact with the fibers of the libriform, the walls of the vessels do not have pores.

Studies of ash wood have shown that the vessels in the trunk, deviating from the vertical in tangential and partly in radial directions, communicate with neighboring vessels through numerous bordered pores and perforation plates. Thanks to these final and intermediate contacts, a single spatially branched water supply system is formed in hardwood. In some breeds, with the formation of a nucleus, the vessels are clogged with tills and put out of action as conductive elements. The tills are outgrowths, in most cases, of adjacent cells of the medullary rays and, rarely, of the woody parenchyma; they have the form of bubbles with lignified walls. The ingrowth of parenchymal cells into the vessel occurs through the pores on its walls (see Fig. 22).

In some breeds, tills form normally after one or more years of vessel operation; Thus, in white locust and pistachio, large vessels are partially clogged with tills already at the end of the first year of existence. In many species, the vessels of the core are usually clogged with tills (in oak, elm), but in certain cases, strong till formation is observed in non-core species (for example, in the false core of beech). The role of tills in a growing tree can be different: tills clog waterways; filling the vessels of the core with tills, especially thick-walled ones (for pistachio), increases the hardness of the wood; if the till cells are alive, they play the role of storage elements along with the woody parenchyma. In a felled tree, the presence of till makes it very difficult to impregnate the wood; for example, the false core of beech is almost impossible to impregnate. Tracheids in hardwoods can be of two types: vascular and fibrous (Fig. 23). Vascular tracheids are predominantly water-conducting elements, the length of which rarely exceeds 0.5 mm; in their shape, size, and also in the location of the pores, they are similar to segments of small vessels; their walls are often equipped with spiral thickenings. The vascular tracheid can be considered as an intermediate element between a typical tracheid and a vessel segment.

The fibrous tracheid, in turn, is a transitional element from the tracheid to the libriform fiber; it has the form of a rather long fiber with pointed ends, a thick shell and a small cavity; the pores on the walls are small, bordered, mostly with a hole slotted shape. Fibrous tracheids differ from libriform fibers in a somewhat smaller wall thickness, but mainly in the presence of clearly bordered pores, while libriform fibers have simple pores. Tracheids are found in the wood of not all hardwoods; tracheids of both types are found in oak wood, where they are confined to the late zone of annual layers; fibrous tracheids are found in pear and apple wood.

Libriform is the main component of hardwood; in some breeds, it occupies up to 76% of the total volume. Libriform fibers are spindle-shaped prosenchymal cells with thick lignified walls (see Fig. 23), a small cavity and a minimum number of simple pores on the walls; from the side, the pores are visible as narrow slits arranged in a spiral (oblique slit-like pores). In most cases, the pointed ends of the libriform fibers are smooth, but in some breeds they are split or have notches (in beech, eucalyptus), resulting in a tighter connection of the fibers to each other. The length of the libriform fibers ranges from 0.3 to 2 mm, and the thickness - from 0.02 to 0.05 mm.

Rice. 23. Elements of hardwood: a - vascular tracheid; b - fibrous tracheid; c - libriform fiber; g - fiber of cloisonne libriform; e - strand of wood parenchyma; e - spindle-shaped cell of woody parenchyma; g - cells of the core rays.

Fully formed libriform fibers are devoid of living content, and their cavities are filled with air. The walls of libriform fibers are strongly thickened in hard wood (oak, ash, beech, hornbeam, etc.) and weaker in soft wood (linden, poplar, willow). On fig. 24 shows a libriform with different wall thicknesses. In some species, such as maples, there are fibers with less thickened walls and live contents; these elements can be considered as intermediate between the fibers of the libriform and the spindle cells of the woody parenchyma.

Along the radius of the trunk, the dimensions of the libriform fibers and the thickness of their walls increase in the direction from the core to the cortex, reach a maximum, after which they remain unchanged or slightly decrease. Along the height of the trunk, the length of the fibers of the libriform and the thickness of their walls decrease in the direction from the butt to the top. The density and strength of hardwood wood depend on the amount of libriform and the size of individual fibers, mainly on the thickness of their walls. The dimensions of the libriform fibers depend on the growth conditions: with the improvement of these conditions, the length of the fibers and the thickness of their shells increase. Thinning causes an increase in the number and length of libriform fibers.

Rice. 24. Fragments of cross-sections of poplar wood (left), beech (middle) and ironwood (right): 1 - vessels; 2- core beam; 3, 4 and 5 - libriform fibers with thin, medium thickness and very thick walls.

In the wood of some species (for example, teak), the so-called cloisonne libriform is found (see Fig. 23). Its fibers, after the end of growth in length and thickening of the shells, are divided by transverse partitions into a number of sections; partitions remain thin and do not become woody. Thus, the fiber of the cloisonne libriform somewhat resembles a strand of woody parenchyma, from which it differs in the nature of the pores and the thickness of the side (longitudinal) walls; in addition, the cavities of the cloisonne libriform have no contents. core rays. Parenchymal cells in hardwoods, as well as in coniferous wood, form primarily core rays, which are much more developed in hardwoods than in conifers. They consist exclusively of parenchymal cells, somewhat elongated along the length of the beam, with thin lignified walls and numerous simple pores, especially in those places where the cells of the beam touch the vessels or tracheids.

In width, the core rays of hardwoods have from one (ash) to several tens (wide rays of oak, beech) rows of cells, and in height - from several rows (boxwood) to several tens and even hundreds of rows of cells (oak, beech). On a tangential section, single-row rays are represented by a vertical chain of cells, and multi-row ones look like a spindle or lentil. The structure of the false-wide beam mentioned above is shown in Fig. 25.

Rice. Fig. 25. Core beam on a radial section of willow wood (left), and a tangential section of hornbeam wood (right): 1 - standing cells; 2 - recumbent cells; 3 - vessel; 4 - false wide beam; 5.6 - narrow beams; 7 - libriform.

In some species (willows), the marginal cells, i.e., the upper and lower rows along the height of the beam, are elongated across the beam and are called erect (Fig. 25); such rays are called heterogeneous rays, in contrast to homogeneous rays, in which all cells are identical in shape. The width of the cells of the medullary rays in the wood of the summer oak is 15 µm, and the height is 17 µm; length of cells in narrow rays 50-55μ, in wide rays 69-94μ. The median (in height) cells of the medullary rays in both deciduous and coniferous species are accompanied on both sides by narrow, air-filled intercellular passages penetrating the ray along its entire length and adjoining the lenticels of the cortex through the intercellular spaces of the cortical parenchyma; through these passages, gas exchange with the atmosphere surrounding the tree is carried out. The cells of the medullary rays in hardwoods can remain alive for a long time; Thus, live cells were found near the core of an apple tree at a 24-year-old, a beech - a 98-year-old, and a hornbeam - even a 107-year-old.

Wood parenchyma. Hardwoods that shed their leaves for the winter need more reserve nutrients than softwoods to produce leaves at the start of the next growing season. As a result, in deciduous species, along with a greater content (volume) of core rays, the woody parenchyma develops more strongly, which is almost absent in coniferous species. Wood parenchyma cells are arranged in vertical rows and provided with simple pores; the terminal cells have a pointed shape, due to which the entire row gives the impression of a fiber divided into sections by transverse partitions (see Fig. 23). Such rows of parenchymal cells are called strands of woody parenchyma. In some species (birch, linden, willow) there are spindle-shaped parenchyma cells (fusiform parenchyma) without transverse partitions. The fusiform parenchyma differs from the tracheid in the type of pores and the absence of spiral thickenings, and from the fibers of the libriform in the thickness of the walls, the type of pores, and the shape of the endings.

Wood parenchyma in hardwoods takes from 2 to 15% of the total volume of wood. In some tropical species, the woody parenchyma forms the bulk of the wood; such breeds give especially light wood (for example, balsa). The distribution of woody parenchyma in the annual layer depends on the species and is of great diagnostic value. There are the following main types of distribution of woody parenchyma: scattered (diffuse) parenchyma, when its cells are more or less evenly distributed over the annual layer (birch, beech, etc.); border (terminal) parenchyma, when the annual layer ends with one or more rows of woody parenchyma (willow, maples, etc.); tangential (metatracheal) parenchyma, when its cells form tangential rows in the late zone of annual layers (oak, walnut, etc.); perivascular (vasicentric) parenchyma, when its cells are grouped near the vessels. The approximate content of various elements in hardwood can be illustrated by the data in Table. 6.

Introduction It is difficult to name any branch of the national economy where wood was not used. in one form or another, and list products in which wood is not integral part. In terms of the volume of use and variety of applications in the national economy, no other material can be compared with it. Wood is used for the manufacture of furniture, joinery and building products. Elements of bridges, ships, bodies, wagons, containers, sleepers, Sports Equipment, musical instruments, matches, pencils, paper, household items, toys, souvenirs. Natural or modified wood is used in engineering and mining; she is feedstock for the pulp and paper industry, the production of wood-based panels. Wood- is a product of plant origin, chemical composition is a complex complex, consisting mainly of organic substances of different composition and structure. Cellulose, hemicellulose and lignin are the most significant for the characteristics of plant raw materials, the content of extractive substances, uronic acids, ash components, as well as the carbohydrate composition of hydrolysates formed during the quantitative hydrolysis of easily and difficultly hydrolysable polysaccharides and other substances is essential. Determination of these components and leads to the most complete characterization of the chemical composition of plant tissue. Lately coniferous wood species are widely used in the wood chemical and wood processing industries, the study of their chemical composition, structure and morphological features plays an important role in the correct and rational use of wood species.1. Analytical review Pine(Pinus L.) is an evergreen tree from the pine family (Pinuseae Lindl). Currently, there are about a hundred species belonging to this genus, of which fourteen grow in Russia and about ninety more were introduced, subspecies of some pines are listed in the Red Book. The most common type of pine growing in Russia is Scotch pine.

Scotch pine (Pinus silvestris L.) is widespread throughout Russia. This is one of the most valuable conifers in our country. A tree of the first size, reaching a height of (35-40) m, evergreen, monoecious, dioecious, anemophilous (wind-pollinated). Under unfavorable conditions, for example, in a swamp, the pine remains a dwarf, and hundred-year-old specimens sometimes do not exceed a height of one meter. Very light-loving tree species. The crown of young trees is cone-shaped, later it is rounded, wider, and in old age it is umbrella-shaped or flat. Very cold and heat resistant. The lifespan of a tree is from 150 to 200 (sometimes 400) years. Propagated by seeds. It has a plastic root system that develops in accordance with the nature and structure of the soil. Usually, four types of root systems are distinguished for Scots pine, which differ quite a lot in shape and structure.

1. A powerful root system with a developed taproot (“radish”) and lateral roots is typical for soils that are fairly fresh and well-drained.

2. A powerful root system with a poorly developed tap root, but exceptionally strongly developed lateral roots, located at an insignificant depth parallel to the soil surface, is typical of dry soils with a very deep groundwater horizon.

3. A poorly developed root system, consisting only of superficially located short, sparsely branched roots, is typical for soils with excessive moisture, semi-marsh and marsh.

4. Dense but shallow "brushed" root system - typical of dense soils with deep water tables.

This plasticity of the pine root system makes it an extremely valuable tree species in terms of silviculture, providing an opportunity for artificial afforestation on the driest, poorest and waterlogged soils.

The trunk of a pine growing in relatively close plantations is slender, straight, even, highly delimbed; in sparse plantations or in the open, the tree is less tall, the trunk is tapered and more gnarled. The bark in different parts of the tree is of different thickness and different colors: in the lower part of the trunk it is thick, furrowed, red-brown, almost gray; in the middle and upper parts of the trunk and on large branches - yellowish-red, peeling off in thin plates, almost smooth, thin. The buds are reddish-brown, oblong-ovate, pointed, 6-12 mm long, mostly resinous, located at the end of the shoot whorled around the terminal bud, sometimes the buds appear on the shoots from the side, but do not form branches. It gives the greatest increase in height under favorable conditions at the age of (15-30) years, reaching 30 m by the age of eighty.

Pine wood with a pink or brown-red heart and yellowish-brown sapwood, straight-grained, light, resinous, durable, easy to process. The annual layers are clearly visible, the early part of the annual layer is light, the late part is dark.

The needles are dark green, growing in bunches of two, 4-7 cm long, convex above, flat below, hard, pointed. Keeps on a tree for three years, falls off along with a shortened shoot. Shortened shoots are arranged in a spiral, evenly covering both the main and side shoots and giving them radial symmetry. Shortened shoots emerge from the axils of the scales, which are reduced leaves. These scales are clearly visible only on a young shoot. The shortened shoot has a complex structure, clearly visible immediately after bud break. It consists of a very short 1 to 2 mm stem, two needles, between which there is a small dormant bud on the stem. In addition, the shortened shoot also has membranous scales of two types, tightly covering it in the form of a tube - the so-called sheath of the shortened shoot. These membranous scales are reduced leaves. They are clearly visible only in spring on young shoots, later they dry up and fall off. The sleeping bud falls along with the needles. If the needles are severely damaged, for example, by insects or if the upper part of the elongated shoot is broken, if the apical bud is damaged, dormant buds germinate in many shortened shoots, and an elongated shoot appears between the two needles. The needles fall from the tree every year, but not all at once, but partially, since individual needles live (2-3) years. Pine needles can serve as a source of vitamin preparations. It was widely used during the Great Patriotic War for the prevention of treatment of hypo- and beriberi.

At the end of May, the pines begin to bloom. At this time, you can see whole clouds of "yellow dust" rising above the forest. In the event of rain, all this pollen falls to the ground and is carried by water to the lowlands, which gives ignorant people reason to talk about the fall of "sulphurous rain." On some branches, male cones are formed, collected in large numbers in the form of a spike-shaped "inflorescence", yellow in color, and on the tops of young shoots of the same tree there are female cones. Female cones are oval in shape, 5 to 6 mm long, reddish during flowering, sitting on short legs. Pollination occurs in the spring, and fertilization in the summer of the following year. Mature pine cones are oblong-ovate, 2.5-7.0 cm long and 2-3 cm wide, brownish-gray, dull, with dense woody seed scales, hanging down on curved legs. Scutes, or apophyses, at the ends of the seed scales are dull or slightly shiny, almost rhombic, the umbilicus (tubercle of the apophysis) is slightly convex. There are cones red-brown, lilac-brown, gray, gray-green. Approximately 85% of the total nut harvest in Russia comes from Siberian pine. In years with an average yield, the raw material reserve of nuts is 733 thousand tons, of which 672 thousand tons are in the Siberian, 43 thousand tons in the Urals and 18 thousand tons in the Far Eastern Federal District. The highest density of the nut harvest falls on the Tomsk region, the Republic of Tyva and the Irkutsk region.

The yield of Siberian pine depends on the conditions of its growth. In the center of the range, only one year out of five is lean, while at the northern border of the range, good and average harvests occur three to four times in 10 years. The average long-term yield of Siberian stone pine forests ranges from 10 to 170 kg/ha. In sparse forests from 140 to 180 years old, yields reach 800 kg/ha.

Seeds are oblong-ovate, 3-4 mm long, of various colors (variegated, gray, black) with a wing three to four times longer than seeds, covering the seed on both sides, like tweezers, and easily separated from it. The seed departure time is extended and lasts from the first days of spring until the end of May - beginning of June. Seed germination and emergence of seedlings is possible throughout the growing season. In the forest, pine begins to bear fruit from the age of forty, in a free state from (15-30) years. Seed years repeat in two-three-five sometimes even up to twenty years (depending on the region and weather conditions).

Seedlings usually with (4-7) trihedral cotyledons. Needles on seedlings are single, sit spirally. Paired needles appear in the second year. The tip of the elongated shoot of the second year ends with one apical and several lateral buds, from which the first whorl is formed in the spring of the next year. Consequently, when determining the age of young pines, two units must be added to the number of whorls, since the whorls are not formed for the first two years. It is relatively easy to determine the age of a pine by whorls up to (40-50) years, since with age the branches of the lower whorls die off and become invisible on the trunk, overgrown with wood and bark. In addition, under favorable conditions during the growing season, pine can produce two or more growths in one year, respectively, forming two or more whorls.

Pine forms a number of forms that differ in the color of the cones, the shape of the apophysis, and the structure of the crowns. Scotch pine has forms with a pyramidal and weeping crown, with golden, silvery and whitish needles in young shoots, with lamellar and scaly bark.

The distribution of the pine area in Siberia covers an area of about 5.7 million km2 south of 66°N. Farthest to the north, it penetrates along the valley of the Lena River (up to about 68 ° N). The largest areas of highly productive pine forests are concentrated in the Angara river basin, in the upper reaches of the Podkamennaya Tunguska, the Irtysh and the Ob.

Siberian pine is not very demanding on fertility and soil moisture (mesoxerophyte, oligotroph). It can grow in extremely dry soils, where not only other tree species, but even herbaceous plants cannot grow. On dry and poor soils, it often forms pure stands - pine forests. On fertile soils, it is usually part of mixed forests.

The trunk and branches of the pine tree are pierced with resin passages filled with resin, which is usually called "resin", it is of great importance for the tree: it heals the wounds inflicted on it, repels pests. The resin is obtained by tapping. Used to obtain turpentine, rosin, etc. The "resinous air", rich in ozone and free from microbes, in pine forests has long been famous for its beneficial properties for human health. In medicine, pine buds are widely used, collected in the spring before they bloom. The kidneys contain resins, essential oils, starch, bitter and tannins. Scots pine needles contain large amounts of vitamin C and carotene. Pine forests, due to the exceptional value of pine wood, are the main object of forest exploitation.

1.1 The structure of wood

Coniferous wood consists of early and late tracheids, core rays, resin canals, wood parenchyma. Macrostructure refers to the structure of wood and wood, visible to the naked eye or through a magnifying glass, and microstructure - visible under a microscope. Usually, three main sections of the trunk are studied: transverse (end), radial, passing through the axis of the trunk, and tangential, passing along the chord along the trunk. When considering sections of a tree trunk with the naked eye or through a magnifying glass, the following main parts of it can be distinguished: bark, cambium, wood and core. The core consists of cells with thin walls, loosely connected to each other. The core, together with the wood tissue of the first year of tree development, forms a core tube. This part of the tree trunk rots easily and has low strength. The bark consists of the skin or rind, cork tissue and bast. The bark or skin protects the tree from harmful environmental influences and mechanical damage. The bast conducts nutrients from the crown to the trunk and roots. Under the bast layer of a growing tree there is a thin annular layer of living cells - the cambium. Every year, during the vegetative period, the cambium deposits bast cells towards the bark and wood cells in a much larger volume inside the trunk. Cell division of the cambial layer begins in spring and ends in autumn. Therefore, the wood of the trunk (part of the trunk from the bast to the core) in a cross section consists of a number of concentric, so-called growth rings, located around the core. Each ring consists of two layers: early (spring) wood, formed in spring or early summer, and late (summer) wood, which forms in late summer. Early wood is light and consists of large, but thin-walled cells; late wood is darker in color, less porous and has great strength, as it consists of small-cavity cells with thick walls. In the process of tree growth, the walls of the wood cells of the inner part of the trunk adjacent to the core constantly change their composition, become woody and impregnated with resin in coniferous species, and tannins in deciduous species. The movement of moisture in the wood of this part of the trunk stops and it becomes stronger, harder and less capable of decay. This part of the trunk, consisting of dead cells, is called the core in some species, in others it is called ripe wood. The part of the younger trunk wood closer to the bark, in which there are still living cells that ensure the movement of nutrients from the roots to the crown, is called sapwood. This part of the wood has a high moisture content, rots relatively easily, has little strength, and has a large shrinkage and a tendency to warp. In the wood of all species, there are core rays that serve to move moisture and nutrients in the transverse direction and create a supply of these substances for the winter. The wood easily splits along the core rays, but it cracks along them when it dries.

In most conifers, mainly in the layers of late wood, there are resinous passages - intercellular spaces filled with resin. In hardwood, there are small and large vessels in the form of tubules running along the trunk. In a growing tree, moisture moves through the vessels from the roots to the crown. Conifers do not have vessels; their functions are performed by elongated closed cells called early tracheids. The mechanical function is performed by late tracheids, which are formed in the second half of the growing season. Movement in the horizontal direction and storage of reserve nutrients during the rest period occurs along the parenchymal cells that make up the medullary rays. Parenchymal cells are also an element in the structure of resin canals and woody parenchyma.

Tracheids make up 90 to 95% of the volume of coniferous wood. Typically prosenchymal cells, have the form of strongly elongated fibers with obliquely cut ends. Tracheids are dead cells; in the trunk of a growing tree, only the newly formed (last) annual layer contains living tracheids. Their death begins in spring, more and more tracheids die by autumn, and by the middle of winter all tracheids of the last annual layer die off.

The cross-sectional shape of tracheids can be rectangular, sometimes square, five- or hexagonal. The dimensions of tracheids in the tangential direction are practically the same in all rocks and range from 27 to 32 µm. The size of early tracheids in the radial direction is twice as large as that of later ones and ranges from 21 to 52 µm. The length of tracheids of domestic breeds is from 2.5 to 4.5 mm. The early and late tracheids almost do not differ in length. Growing conditions affect the size of tracheids; in good conditions, their length and thickness increase. In the annual layer, the tracheids are arranged in regular radial rows. The early tracheids that make up the early zone of the annual layer have thin walls and large internal cavities; in late tracheids, which make up the late zone of the annual layer, the walls are thick, the internal cavities are small. Within one annual layer, the transition from early to late tracheids is gradual. A characteristic feature of tracheids is the presence of bordered pores located mainly on the radial walls at the ends of the tracheids. The number of pores in early and late tracheids is different. In later tracheids, the pores are smaller and much less numerous. Early tracheids have 70 to 90 pores per tracheid, late tracheids have 8 to 25 pores. Studying the structure of wood under a microscope, you can see that its bulk is made up of spindle-shaped cells, elongated along the trunk. A number of cells are elongated in a horizontal direction, that is, across the main cells. Groups of cells identical in form and function are combined into tissues that have different purposes in the life of wood: conductive, storage, mechanical. A living cell has a membrane, protoplasm, cell sap and a nucleus. Cell membranes are composed of several layers of very thin fibers called microfibrils, which are compactly stacked and directed in spirals at different angles to the longitudinal axis of the cell in each layer. Sometimes microfibrils are oriented in opposite spirals. The microfibril consists of long filamentous chain molecules of cellulose, a high molecular weight natural polymer with a complex structure of macromolecules. Cellulose macromolecules are elastic and highly elongated. The cell membrane also contains other organic substances - lignin and hemicellulose, which are located mainly between microfibrils, as well as a small amount of inorganic substances in the form of alkaline earth metal salts.

Table 1 - The content of various elements in coniferous wood

1.2 Chemical composition

Chemical composition certain types tree species, as well as their parts, is qualitatively similar, but there are significant differences in the quantitative content of individual components. There are also individual characteristics in the quantitative content of individual components within one species, associated with age and growing conditions. Wood is made up of organic substances, which include carbon, hydrogen, oxygen and some nitrogen. Absolutely dry pine wood contains on average: 49.5% carbon; 6.1% hydrogen; 43.0% oxygen; 0.2% nitrogen.

In addition to organic substances, wood contains mineral compounds that produce ash during combustion, the amount of which varies between (0.2-1.7)%; however, in some species (saxaul, pistachio kernels), the amount of ash reaches (3--3.5)%. In the same breed, the amount of ash depends on the part of the tree, position in the trunk, age and growing conditions. More ash is given by the bark and leaves; Branch wood contains more ash than trunk wood; for example, birch and pine branches produce 0.64 and 0.32% ash during combustion, and stem wood - 0.16 and 0.17% ash. The wood of the upper part of the trunk gives more ash than the lower; this indicates a high ash content in young wood.

The composition of the ash includes mainly salts of alkaline earth metals. Pine, spruce and birch wood ash contains over 40% calcium salts, over 20% potassium and sodium salts, and up to 10% magnesium salts. Part of the ash from 10 to 25% is soluble in water (mainly alkalis - potash and soda). In former times, K2CO3 potash, used in the production of crystal, liquid soap and other substances, was extracted from wood ash. Ash from the bark contains more calcium salts (up to 50% for spruce), but less salts of potassium, sodium and magnesium. The main chemical elements (C, H and O) included in the composition of wood and the above-mentioned basic chemical elements form complex organic substances.

The most important of them form a cell membrane (cellulose, lignin, hemicelluloses - pentosans and hexosans) and make up 90--95% of the mass of absolutely dry wood. The remaining substances are called extractive, that is, extracted by various solvents without a noticeable change in the composition of the wood; of these, tannins and resins are the most important. The content of basic organic substances in wood depends to some extent on the species. This can be seen from table 2

Table 2 - The content of organic substances in wood of different species

On average, it can be assumed that coniferous wood contains (48--56)% cellulose, (26--30)% lignin, (23--26)% hemicelluloses containing (10--12)% pentosans and about 13 % hexosans; at the same time, hardwood contains (46--48)% cellulose, (19--28)% lignin, (26--35)% hemicelluloses containing (23--29)% pentosans and (3--6 ) % hexosans. Table 2 shows that coniferous wood contains an increased amount of cellulose and hexosans, while hardwood wood is characterized by a high content of pentosans. In the cell membrane, cellulose is in combination with other substances. A particularly close relationship, the nature of which is still not clear, is observed between cellulose and lignin. Previously, it was believed that lignin was only mechanically mixed with cellulose; however, in recent years, more and more people have come to believe that there is a chemical bond between them.

The chemical composition of early and late wood in the annual layers, that is, the content of cellulose, lignin and hemicelluloses, is almost the same. Early wood contains only more substances soluble in water and ether; this is especially true for larch. The chemical composition of wood varies little along the height of the trunk. So, in the composition of oak wood, no practically tangible differences were found in the height of the trunk. In pine, spruce and aspen at the age of maturity, a slight increase in the content of cellulose and a decrease in the content of lignin and pentosans in the middle part of the trunk were found. The wood of pine, spruce and aspen branches contains less cellulose (44--48)%, but more lignin and pentosans. However, no noticeable differences in the chemical composition of the wood of the trunk and large branches were found in the oak, only in small branches were found less tannins (8% in the trunk and 2% in the branches). The difference in the chemical composition of sapwood and summer oak heartwood can be seen from the data in Table 3.

Table 3 - The difference in the chemical composition of sapwood and pine kernel wood

As we see from the table, a noticeable difference was found only in the content of pentosans and tannins: there are more of them in the wood of the kernel (and less ash). The chemical composition of the cell membranes of the cambium, newly formed wood and sapwood varies greatly: the content of cellulose and lignin sharply increases in the elements of wood (in ash from 20.2 to 4.6% in cambium, to 58.3 and 20.9% in sapwood). ), but the content of pectins and proteins also sharply decreases (from 21.6 and 29.4% in cambium to 1.58 and 1.37% in sapwood). The influence of growing conditions on the chemical composition of wood has been little studied.

Cellulose is a natural polymer, a polysaccharide with a long chain molecule. The general formula of cellulose is (C6H10O5) n, where n is the degree of polymerization from 6000 to 14000. It is a very stable substance, insoluble in water and common organic solvents (alcohol, ether and others), white in color. Bundles of cellulose macromolecules - the thinnest fibers are called microfibrils. They form the cellulose framework of the cell wall. Microfibrils are oriented mainly along the long axis of the cell, between them there is lignin, hemicelluloses, and also water. Cellulose consists of long chain molecules formed by repeating units consisting of two glucose residues. Each pair of glucose residues linked together is called a cellobiose. Glucose residues are formed after the release of a water molecule when glucose molecules are combined during the biosynthesis of cellulose polysaccharide. In cellobiose, glucose residues are rotated by 1800, the first carbon atom of one of them is connected to the fourth carbon atom of the neighboring unit.

Considering cellulose at the molecular level, we can say that its macromolecule has the form of an elongated non-planar chain formed various structures links. The presence of various units is associated with weak intramolecular bonds between hydroxyl groups (OH-OH) or between a hydroxyl group and oxygen (OH-O).

Cellulose has 70% crystalline structure. Compared to other linear polymers, cellulose has special properties, which is explained by the regularity of the macromolecule chain structure and significant forces of intra- and intermolecular interaction.

When heated to the decomposition temperature, cellulose retains the properties of a glassy body, that is, it is characterized mainly by elastic deformations. Cellulose is a chemically stable substance; it does not dissolve in water and most organic solvents (alcohol, acetone, etc.). Under the action of alkalis on cellulose, physicochemical processes of swelling, rearrangement and dissolution of low molecular weight fractions proceed simultaneously. Cellulose is not very resistant to the action of acids, which is due to glucosidic bonds between the elementary units. In the presence of acids, hydrolysis of cellulose occurs with the destruction of chains of macromolecules. Cellulose is a white substance with a density of 1.54 to 1.58 g/cm3.

The concept of hemicellulose combines a group of substances that are similar in chemical composition to cellulose, but differ from it in the ability to easily hydrolyze and dissolve in dilute alkalis. Hemicelluloses are mainly polysaccharides: pentosans (C5H8O4)n and hexosans (C6H10O5)n with five or six carbon atoms in the main unit. The degree of polymerization of hemicelluloses (n = 60-200) is much less than that of cellulose, i.e., the chains of molecules are shorter. During the hydrolysis of hemicellulose polysaccharides, simple sugars (monosaccharides) are formed; hexosans are converted to hexoses, and pentosans to pentoses. Usually, hemicelluloses are not obtained from wood in the form of marketable products. However, in the chemical processing of wood, they are widely used to obtain many valuable substances. For example, when wood is heated with twelve percent hydrochloric acid, almost all pentosans (93-96)% are converted into simple sugars - pentoses - and after the removal of three water molecules from each monosaccharide molecule, furfural is formed - a product widely used in industry. In a growing tree, hexosans are reserve substances, and pentosans perform a mechanical function.

In addition to carbohydrates (cellulose and hemicellulose), the cell wall contains an aromatic compound, lignin, which has a high carbon content. Cellulose contains 44.4% carbon, and lignin (60--66)%. Lignin is less stable than cellulose, and easily goes into solution when wood is treated with hot alkalis, aqueous solutions of sulfurous acid or its acidic salts. This is the basis for obtaining technical cellulose. Lignin is obtained in the form of waste during the cooking of sulfite and sulfate pulp, during the hydrolysis of wood. The lignin contained in black alkalis is mainly burned during regeneration.

Lignin is used as a pulverized fuel, a substitute for tannins, in the production of molding earth binders (in the foundry industry), plastics, artificial resins, for the production of activated carbon, vanillin, and others. However, the question of the full qualified chemical use of lignin has not yet been resolved. Of the remaining organic substances contained in wood, resins and tannins have received the greatest industrial use.

Resin refers to hydrophobic substances soluble in neutral non-polar solvents.

This group of substances is usually divided into water-insoluble resins (liquid and solid) and gum resins containing water-soluble gums. Among liquid resins, the most important is resin, which is obtained from wood (sometimes from the bark) of conifers as a result of tapping. The tapping of pine and cedar is carried out as follows. In autumn, a vertical groove is made with special tools on a section of the trunk cleared of coarse bark, and with the onset of warm weather in the spring, strips of bark and wood directed at an angle of 30 ° to the groove are systematically removed and the so-called shovels are formed. The depth of the warp is usually (3--5) mm. The wound inflicted on a tree by tapping is called a karra.

From the cut resin passages, the resin, which is under pressure (10-20) atmospheres, flows into the shoes and goes along the groove to the receiver. After applying four to five new pieces, the resin is selected from the conical receiver with a steel spatula. To increase the yield of resin, chemical stimulants (chlorine or sulfuric acid) are used, which are used to treat the freshly opened wood surface.

Spruce tapping is carried out by applying carr in the form of narrow longitudinal strips. To obtain resin from larch, channels are drilled deep into the trunk until they encounter large resin "pockets", which often form in the lower part of the trunk. Larch resin is highly valued and is used in the paint and varnish industry for the manufacture of the best varieties of varnishes and enamel paints. Fir resin is extracted from the "blisters" that form in the bark. The resin from the pierced "blisters" is squeezed out into portable receivers. Fir resin resembles Canadian balsam in its properties and is used in optics, microscopic technology, and the like.

Pine resin is extracted in the largest quantities, which is a transparent resinous liquid with a characteristic pine smell. In the air, resin hardens and turns into a brittle whitish mass - barras. The pine resin obtained as a result of tapping contains approximately 75% rosin and 19% turpentine, the rest is water. Gum can be considered as a solution of solid resin acids (rosin) in liquid turpentine oil (turpentine). Recycling resin is carried out at rosin-turpentine plants and consists in distillation with water vapor of the volatile part - turpentine. The remaining non-volatile part is rosin.

Turpentine and rosin can be obtained by extraction processing of stump resin - the heart part of pine stumps, enriched with resin due to rotting of low-resin sapwood. Gasoline is most often used as a solvent. The resulting extract is distilled. The solvent and turpentine are distilled off, and the rosin remains. Extraction products are inferior in quality to turpentine and rosin obtained from resin. Turpentine is widely used as a solvent in the paint and varnish industry, for the production of synthetic camphor and other products. Camphor is used in large quantities as a plasticizer in the production of celluloid, varnishes and film.

The main consumer of rosin is the soap industry, where it is used to make laundry soap. In large quantities, rosin glue is used for sizing papers. Glycerin ester of rosin is introduced into the composition of nitro-varnishes to give the film shine. Rosin is used for the preparation of electrical insulating materials, in the production of synthetic rubber, etc. Larch gum is of great industrial importance. Gum is extracted from crushed wood with acidic water (acetic acid concentration 0.2%) at a temperature of 30 °. After evaporation to a concentration of (60--70)%, a commercial product is obtained. It is applied in textile industry for the manufacture of paints, in the printing, paper industry.

The concept of tannins or tannins combines all substances that have the properties of tanning raw leather, giving it resistance to decay, elasticity, and the ability not to swell. The most rich in tannins is the wood of the oak core from 6 to 11% and chestnut from 6 to 13%. The bark of oak, spruce, willow, larch and fir contains from 5 to 16% tannins. The growths on oak leaves - galls contain from 35% to 75% tannins (one of the varieties of tannins). In the leaves and roots of bergenia, the content of tannins is (15-25)%.

Tannins are soluble in water and alcohol, have an astringent taste, when combined with iron salts they give a dark blue color, and are easily oxidized. Tannins are extracted with hot water from crushed wood and bark. The marketable product is either a liquid or dry extract, which is obtained after the solution has been evaporated in a vacuum apparatus and dried. Essential oils, lactoresins and dyes can also be obtained from woody plants.

Essential oils belong to the group of terpenoids (isoprenoids) - hydrocarbons built from a different number of isoprene units.

From the needles and cones of different types of fir, fir oil is extracted, which is a transparent, colorless aromatic liquid that quickly evaporates in air. The needles of the Siberian fir contain from 0.63 to 3%, and the needles of the Caucasian fir 0.2% fir oil. Fir oil has applications in pharmaceutical production, in perfumery and for the preparation of varnishes. Volatile essential oils of coniferous species of pine, spruce, western arborvitae, have the properties of phytoncidity, i.e., the ability to kill microbes in the air or in water.

Pine buds contain essential oil, resins, starch, tannins, pinipicrin. The needles contain a lot of ascorbic acid, tannins, and also contain alkaloids, essential oil. Gum contains up to 35% essential oil and resin acids. In medicine, pine buds are used in the form of infusion, tincture, decoction, extract as an expectorant, diuretic, disinfectant, anti-inflammatory and antiscorbutic agent. Pine buds are an integral part of the breast collection; in combination with coniferous needles in the form of infusion and extract, they can be used to prepare coniferous baths. Polyprenol -- active ingredient pine needles has an antiserotonergic effect. Coniferous needles are used to prepare concentrates and infusions used for scurvy, as well as for therapeutic baths. Pine bud extract has bactericidal properties against staphylococcus, shigella and Escherichia coli. Turpentine is part of the ointments, liniments used for neuralgia, myositis, for rubbing. It is prescribed orally and for inhalation for bronchitis, bronchiectasis. Tar has disinfectant and insecticidal properties, has a local irritant effect. It is used in the form of ointments to treat skin conditions and wounds. The bark contains tannins. Gum from the bark of cedar pine contains turpentine and rosin.

Lactoresins are the milky juices of some plants, close to resins. These include rubber and gutta-percha. Rubber is extracted from the bark of the Hevea brasiliensis tree and is a yellow to dark amorphous mass soluble in carbon disulfide, chloroform, ether and turpentine. Gutta-percha is obtained from some tropical tree species (for example, Isonandra gutta Hook and others). Of the Russian breeds, gutta-percha is contained in the root bark (up to 7%) of the warty and European euonymus. Purified gutta-percha is a brown solid mass, easily soluble in carbon disulfide, chloroform and turpentine. It is used to make cliches for drawings, insulation of electrical cables and more.

Coloring substances can be found both in wood and in the bark, leaves and roots. The wood contains dyes of red, yellow, blue and brown. Of the species growing in our country, for dyeing fabrics and yarn yellow, the local population in the Caucasus uses the wood of maclura, mulberry, skumpia, hornbeam bark, sumac and hop hornbeam, for dyeing red - dry buckthorn bark, brown - skumpia wood , walnut peel and more.

The chemical composition of tree bark differs sharply from the chemical composition of wood (xylem). It should also be noted that the inner and outer parts of the bark, which have different functional purposes and, accordingly, the structure, differ significantly from each other in composition. But quite often, the analysis of the chemical composition of the bark is done without dividing it into bast and crust.

A distinctive feature of the chemical composition of the bark is the high content of extractive substances and the presence of certain specific components that cannot be removed by neutral solvents. By successive extraction with solvents with increasing polarity, from 15 to 55% of its mass is extracted from the bark of different species. The next treatment with a 1% NaOH solution additionally dissolves from 20 to 50% of the mass. As a result of such successive treatments, the tree bark loses from 10 to 75% of its own weight. With all this, not only some of the hemicelluloses are removed from the bark, but also such specific components as suberin and polyphenolic acids of the bark, which cannot be classified as extractive substances. The features of the structure and chemical composition of the bark cause certain difficulties in its analysis and require modification of the methods developed for the analysis of wood, namely, the introduction of additional pretreatments with aqueous and alcoholic solutions and sodium foxide. Otherwise, the presence of suberin and polyphenolic acids can lead to a significant overestimation of the results of the determination of holocellulose and lignin. The bark, when compared with wood, contains more minerals (1.5-5.0)%. Sometimes this is due to the deposition of carbonate crystals in the crust. The ash content of the bark largely depends on the growing conditions of the tree (the composition and moisture content of the soil, etc.).

Mass fraction holocellulose in the bark is approximately two times less than in wood, while its content in the bast is higher than in the bark. Cellulose in the bark, as well as in wood, is the main polysaccharide, but unlike wood, it cannot be called the predominant component of the bark. In the literature, values from 10 to 30% are given for the mass fraction of cellulose in unextracted bark samples.

As in wood, the main hemicelluloses in the bark of coniferous species are glucomannans and xylans, while those of hardwoods are xylans. In the walls of cork cells found glucan - callose. Callose also appears in the phloem as a substance that clogs the sieve plates. Attention is drawn to a rather large mass fraction of uronic acids in the bark, especially in the tissues of the bast, which is associated with a high content of pectin substances. This is consistent with a significantly higher amount of water-soluble polysaccharides in the bark compared to wood. The composition of pectin substances in the bark does not differ significantly from the composition of these substances in wood. Note only a higher content of arabinose.

As already emphasized, one should be cautious about the data available in the literature on the determination of lignin and other components in the bark. For example, for frankincense pine (Pinus taeda), the range of results for determining lignin in the bark is very wide: from 20.4 to 52.2%. The differences may be due to the introduction of different methods of preparing bark samples for analysis and conducting the analysis itself.

Lignin in bark tissues is less evenly distributed than in wood. The outer layer of the crust is more lignified than the inner one. The walls of stony cells are the most lignified. Lignin is also found in the walls of fibers and some types of parenchymal cells of the phloem and crust. The distribution of lignin among different types of cells in the cortex has strong species differences. The bark lignin is more condensed than in the wood of the same tree species, which is confirmed to some extent by data on bark delignification. Bark is more difficult to delignify than wood.

A characteristic component of the outer layer of the bark is suberin, a product of copolycondensation, mainly of higher (C16-C24) saturated and monounsaturated aliphatic a, dicarboxylic acids with hydroxy acids (the latter can be additionally hydroxylated). Participation in the polycondensation of monomers with three or more multifunctional groups (carboxylic, hydroxyl) leads to the formation of a polyester with a network structure. Some researchers admit the existence of simple ether bonds. As a result, suberin cannot be isolated from the bark unchanged, since it cannot be extracted with neutral solvents, and ester bonds make it a very labile component. From the bark, suberin is isolated in the form of suberin monomers after saponification with aqueous or alcoholic solutions of alkali and decomposition of the resulting suberin soap with mineral acid.

Suberin is contained in the periderm, including the wound. It is localized in cork cells, being an integral part of the cell wall. The cork tissue of the cork oak contains (42-46)% suberin, the Brazilian tropical tree paosantha (Kielmeyera coriacea) - 45%, and the cork cells of the warty birch - 45% suberin. The mass fraction of suberin in the outer layer of the bark occasionally exceeds (2-3)%, but there are tree species that are characterized by a high content of suberin. In the above tree species, suberic monomers make up (2-40)% of the mass of the outer part of the bark. A characteristic feature of the cork tissue of birch - birch bark is the accumulation along with suberin of triterpene alcohol - betulin. The composition of suberic monomers is very diverse. In addition to the dicarboxylic and hydroxy acids mentioned above, the composition of suberic monomers includes monobasic fatty acids, monohydric higher fatty alcohols (up to 20% by weight of suberin), phenolic acids, dilignols (dimers of phenylpropane units) and others.

As already noted, the treatment of bark previously extracted with neutral solvents with a 1% aqueous solution of NaOH extracts up to (15-50)% of the material, which is a group of phenolic substances with acidic properties. This gave reason to call them polyphenolic acids. However, not carboxyl, but carbonyl groups were found in them. After precipitation from an alkaline solution by acidification with mineral acids, polyphenolic acids become partially soluble in water and polar organic solvents. In all likelihood, "polyphenolic acids" are polymeric substances of the flavonoid type, related to condensed tannins and therefore capable of undergoing rearrangement in an alkaline environment with the appearance of carbonyl groups.

Significant differences in the structure and chemical composition of the bark and wood necessitate the separate processing of these components of wood biomass from both technological and economic points of view. but existing methods removal of bark (barking) is associated with loss of wood. The debarking waste, along with the bark, contains a significant amount of wood, which complicates the chemical processing of such raw materials. The variety of chemical compounds present in the bark makes the idea of extracting the most valuable components attractive. The development of this area of bark utilization is constrained by the relatively low content of extractable components. As a result, the main areas of bark processing are still limited to its utilization as an organic material as fuel, in agriculture, etc. Rare examples of the use of the bark of individual tree species for the extraction of tannins, the production of cork, the production of tar (from birch bark) and the isolation of fir balsam from the bark of growing fir trees, unfortunately, do not improve the overall picture of the inefficient use of valuable organic compounds contained in the bark.

1.3 Physical properties

Physical properties are those properties of wood that are observed when it interacts with wood. external environment and do not lead to a change in the composition and integrity of the wood. These properties are characterized by the appearance of wood (color, gloss, texture), density, humidity, hygroscopicity, heat capacity and others.

1 Properties that determine the appearance of wood. Among these properties, we note its color, luster and texture. The color of the wood is extremely varied. It depends on the type of tree and the climate. As a rule, tree species of the temperate zone are pale in color, while those of the tropical zone are bright. So, the wood of pine, spruce, aspen, birch is weakly colored, and the rocks of the warm zone (oak, walnut, boxwood, white locust) have a more intense color. The color intensity increases with the age of the tree. Wood also changes its color under the influence of light and air. Some types of wood have a sheen. The brilliance of wood depends on the degree of development of the core rays. In the radial section, such species as maple, beech, white locust, mahogany have brilliance. Strongly developed core rays of oak in a radial section give brilliant spots. The texture of wood is a pattern in a radial or tangential section and depends on the structure of the wood. It consists of clearly distinguishable large vessels, wide core rays, annual layers, and the direction of the fibers. The more complex the structure of wood, the more diverse its texture. Oak and beech species have a beautiful texture in the radial section, and ash, chestnut, walnut, oak, larch in the tangential section. The smell of wood depends on the presence of resin, essential oils, tannins and other substances in it. Coniferous species - pine, spruce - have a characteristic smell of resin. Oak has a tannin smell. When freshly cut, wood has a stronger odor than when dried.

2 Hygroscopicity and humidity. Wood, having a fibrous structure and a large porosity from 30 to 80%, has a huge inner surface, which easily collects water vapor from the air (hygroscopicity). The moisture that wood acquires as a result of prolonged exposure to air with a constant temperature and humidity is called equilibrium moisture. It is achieved at the moment when the vapor pressure above the wood surface is equal to the vapor pressure of the surrounding air. According to the moisture content, wet wood is distinguished - with a moisture content of up to 100% or more; freshly cut - 35% and above; air-dry - (15-20)%; room-dry - (8-12)% and absolutely dry wood, dried to constant weight at a temperature of 100-105 °C. Water in wood can be in three states - free, physically bound and chemically bound. Free or capillary water fills the cavities of cells and vessels and intercellular spaces. Bound or hygroscopic water is found in the walls of wood cells and vessels in the form of the thinnest hydrated shells on the surface of the smallest elements that make up the cell walls. The humidity of wood, when the cell walls are saturated with water, and the cavities and intercellular spaces are free of water, is called the chapel of hygroscopic moisture. For wood of various species, it ranges from 23 to 35% (on average 30%) of the mass of dry wood. Hygroscopic water, covering the surface of the smallest particles in the cell walls with water shells, enlarges and pushes them apart. At the same time, the volume and mass of wood increase, and the strength decreases. Free water, accumulating in cell cavities, does not significantly change the distance between wood elements and therefore does not affect its strength and volume, increasing only mass and thermal conductivity.

3 Shrinkage and swelling. Shrinkage of wood with a decrease in its linear dimensions and volume occurs only with the evaporation of hygroscopic moisture, but not capillary moisture. However, when hygroscopic moisture evaporates, a linear contraction occurs and, conversely, when hygroscopic moisture is absorbed, swelling occurs. The shrinkage of wood due to the heterogeneity of its structure in different directions is not the same. Linear shrinkage along the fibers for most tree species does not exceed 0.1%, in the radial direction - (3-6)%, and in the tangential direction - (7-12)%. This is accompanied by the appearance of internal stresses in the wood, which can cause it to warp and crack. Warping can be longitudinal and transverse. When wood swells as a result of the absorption of water that impregnates cell membranes, it increases in volume. The swelling of wood is not the same in different directions: along the fibers (0.1-0.8)%, in the radial direction (3-5)% and tangentially - (6-12)%. When moistened, as a result of saturation of cell membranes with water, wood increases in weight and volume. After further saturation of the wood with water, moisture saturates the cavities of the cells and the spaces between them. In this case, the weight of the wood changes. But the volume does not increase.

4 Density and bulk density. Since the composition of all tree species is dominated by the same substance - cellulose, the density of their wood is approximately the same and averages 1.54 g / cm3. The bulk density of wood of different species, and even of the same species, depends on the structure and porosity of the growing tree, which change with climate, soil, shading, and other natural conditions. In most tree species in an absolutely dry state, it is less than 1 g/cm3. With an increase in humidity, the volumetric mass of wood increases, therefore, the characteristics of wood in terms of volumetric mass are always carried out at the same moisture content. In accordance with GOST, it is customary to determine the volumetric mass of wood at a moisture content of 11-13% at the time of testing, as well as in an absolutely dry state. By volumetric weight at a moisture content of 12%, wood species are divided into groups: low density (540 kg/m3), medium density (550-740 kg/m3.), high density (750 kg/m3).

5 Thermal conductivity. The thermal conductivity of wood is its ability to conduct heat through the entire thickness from one surface to another. The thermal conductivity of dry wood is negligible, which is explained by the porosity of its structure. Thermal conductivity coefficient of wood (0.12-0.39) W / (m * hail). Cavities, intercellular and intracellular spaces in dry wood are filled with air, which is a poor conductor of heat. Due to its low thermal conductivity, wood is widely used in construction. Dense wood conducts heat somewhat better than loose wood. The moisture content of wood increases its thermal conductivity, since water is a better conductor of heat than air. In addition, the thermal conductivity of wood depends on the direction of its fibers and species. For example, the thermal conductivity of wood along the fibers is approximately twice that across.

6 Sound conductivity. The property of a material to conduct sound is called sound conductivity. It is characterized by the speed of sound propagation in the material. In wood, sound travels fastest along the fibers, slower in the radial direction, and very slowly in the tangential direction. The sound conductivity of wood in the longitudinal direction is 16 times, and in the transverse direction it is three to four times greater than the sound conductivity of air. This negative property of wood requires the use of soundproof materials when constructing wooden partitions, floors and ceilings. The sound conductivity of wood and its ability to resonate (to amplify sounds without current distortion) is widely used in the manufacture of musical instruments. The increased humidity of wood lowers its sound conductivity.

7 Electrical conductivity. The electrical conductivity of wood is characterized by its resistance to the passage of electric current. The electrical conductivity of wood depends on the species, temperature, grain direction and humidity. The electrical conductivity of dry wood is negligible, which allows it to be used as an insulating material. With an increase in humidity in the range from 0 to 30%, the electrical resistance drops by a factor of millions, and with an increase in humidity above 30%, by tens of times. The electrical resistance of wood along the fibers is several times less than across the fibers; an increase in the temperature of the wood leads to a decrease in its resistance by about a factor of two.

8 Properties of wood, manifested under the influence of electromagnetic radiation. Surface areas of wood can be effectively heated with invisible infrared rays. Significantly deeper - up to (10-15) cm - rays of visible light penetrate into the wood. By the nature of the reflection of light rays, it is possible to assess the presence of visible defects in wood. Light laser radiation burns through wood and has recently been successfully used to burn parts of complex configuration. Ultraviolet rays penetrate much worse into wood, but they cause a glow - luminescence, which can be used to determine the quality of wood. X-rays are used to determine the features of the fine structure of wood, to identify hidden defects, and in other cases. Of the nuclear radiation, beta radiation can be noted, which are used in the densimetry of a growing tree. Much more widely can be used gamma radiation, which penetrates deeper into the wood and is used to determine its density, detect rot in the mine rack and structures.

1.4 Mechanical properties

Mechanical properties characterize the ability of wood to resist the influence of external forces (loads). According to the nature of the action of forces, static, dynamic, vibrational and long-term loads are distinguished. Static loads are loads that increase slowly and smoothly. Dynamic, or shock, loads act on the body instantly and in full force. Vibratory loads are called, in which both the magnitude and direction change. Long-term loads act for a very long time. Under the action of external forces in the wood, the connection between its individual parts is broken and the shape changes. Due to the resistance of wood to external loads, internal forces arise in the wood. The mechanical properties of wood include strength, hardness, deformability, impact strength.

1 Durability. Strength is the ability of wood to resist irritation under the action of mechanical loads. The strength of wood depends on the direction of the acting loads, the species. It is characterized by tensile strength - the stress at which the sample is destroyed. Only the bound moisture contained in the cell membranes has a significant effect on the strength of wood. With an increase in bound moisture, the strength of wood decreases (especially at a moisture content of (20-25)%. A further increase in moisture beyond the limit of hygroscopicity (30%) does not affect the strength of wood. In addition to moisture, the mechanical properties of wood are also affected by the duration of the load. Therefore, when testing wood, it adheres to a given loading rate for each type of test.There are main types of force actions: tension, compression, bending, shearing.Tensile strength. average value the limit of tensile strength along the fibers for all breeds is 130 MPa. The tensile strength along the fibers is greatly influenced by the structure of the wood. Even a slight deviation from the correct arrangement of the fibers causes a decrease in strength. The tensile strength of wood across the fibers is very low and averages 1/20 of the tensile strength along the fibers, i.e. 6.5 MPa. Therefore, wood is almost never used in parts that work in tension across the fibers. The strength of wood across the fibers is important in the development of cutting modes and wood drying modes. Ultimate compressive strength. Distinguish between compression along and across the fibers. When compressed along the fibers, the deformation is expressed in a slight shortening. Compressive failure begins with buckling of individual fibers; in wet samples and samples from soft and viscous rocks, it manifests itself as a collapse of the ends and buckling of the sides, and in dry samples and in hard wood it causes a shift of one part of the sample relative to the other. The compressive strength of wood across the fibers is about eight times lower than along the fibers. When compressing across the fibers, it is not always possible to accurately determine the moment of destruction of the wood and determine the magnitude of the destruction of the load. The wood is tested for compression across the fibers in the radial and tangential directions. Ultimate strength in static bending. During bending, especially under concentrated loads, the upper layers of wood experience compressive stress, and the lower layers experience tension along the fibers. Approximately in the middle of the height of the element, there is a plane in which there is neither compressive nor tensile stress. This plane is called neutral; the maximum tangential stresses occur in it. The ultimate strength in compression is less than in tension, therefore, the destruction begins in the stretched zone and is expressed in the rupture of the outermost fibers. The tensile strength of wood depends on the species and humidity. In bending, twice the compressive strength along the fibers. Shear strength of wood. External forces that cause the movement of one part of the part relative to another are called shear. There are three cases of shear: shearing along the fibers, across the fibers and cutting. The shear strength along the fibers is 1/5 of the compressive strength along the fibers. The shear strength across the fibers is approximately two times less than the shear strength along the fibers. The strength of wood when shearing across the fibers is four times higher than the strength when shearing along the fibers. Chip resistance of wood. Splitting is the ability of wood under the action of a wedge to be divided into parts along the fibers. The splitting of wood in terms of the action of force and the nature of destruction resembles tension across the fibers, which in this case is eccentric, that is, the result of the action of tension and bending. Tension can take place along the radial and tangential planes. The resistance in the radial plane of hardwood is less than in the tangential plane. This is due to the influence of the core rays. In conifers, on the contrary, chipping along the tangential plane is less than along the radial one. With tangential splitting in conifers, destruction occurs in early wood, the strength of which is much less than the strength of late wood.

2 Hardness. Hardness is the ability of wood to resist the introduction of harder bodies into it. The hardness of the end surface is higher than the tangential and radial ones by 30% in hardwoods and by 40% in conifers. The amount of hardness is influenced by the moisture content of the wood. When the humidity changes by 1%, the end hardness changes by 3%, and the tangential and radial hardness by 2%. According to the degree of hardness, all tree species at 12% humidity can be divided into three groups:

A) soft (end hardness 38.6 MPa or less) - pine, spruce, cedar, fir, poplar, linden, aspen, alder;

B) hard (end hardness from 338.6 to 82.5 MPa) - Siberian larch, birch, beech, elm, elm, elm, maple, apple tree, ash;

C) very hard (end hardness over 82.5 MPa) - white locust, iron birch, hornbeam, dogwood, boxwood.

The hardness of wood is essential when processing it with cutting tools: milling, sawing, peeling, and also in those cases when it is subjected to abrasion when constructing floors, stairs, railings.

3 Wear resistance. Wear resistance - the ability of wood to resist wear, i.e. gradual destruction of its surface zones during friction. Tests for the wear resistance of wood have shown that wear from the side surfaces is much greater than from the surface of the end cut. With an increase in the density and hardness of wood, wear decreased. Wet wood has more wear than dry wood.

4 Ability to hold fasteners. A unique property of wood is the ability to hold fasteners: nails, screws, staples, crutches, etc. When a nail is driven into wood, elastic deformations occur that provide sufficient friction force to prevent the nail from pulling out. The force required to pull out a nail driven into the end of the sample is less than the force applied to a nail driven across the fibers. With increasing density, the resistance of wood to pulling out a nail or screw increases. The effort required to pull out the screws (ceteris paribus) is greater than to pull out the nails, since in this case the resistance of the fibers to cutting and breaking is added to the friction.

5 Ability to bend. The technological operation of wood bending is based on its ability to deform relatively easily under the action of avoiding forces. The ability to bend is higher in ring-vascular species - oak, ash and others, and in scattered-vascular species - beech. Conifers have less ability to bend. Wood is subjected to bending, which is in a heated and wet state. This increases the pliability of wood and allows, due to the formation of frozen deformations during subsequent cooling and drying under load, to fix new form details.

Growing trees have the following components: roots, trunk, branches, leaves. The root system of trees acts as a supplier of moisture and nutrients from the soil through the trunk and branches to the leaves. In addition, the roots hold the trees upright. Through the branches, moisture enters the leaves, in which the process of photosynthesis takes place - the conversion of the radiant energy of the sun into the energy of chemical bonds of organic substances with the absorption of carbon dioxide from the air and the release of oxygen. It is no coincidence that forests are called the lungs of the planet. The products of photosynthesis from the leaves are transferred along the branches to the rest of the trees - the trunk and roots. Thus, the branches act as channels through which the exchange of substances takes place between the leaves and the rest of the tree.

Coniferous trees - pine, cedar, spruce, larch - have narrow leaves - needles, and hardwoods - wide leaves. As a rule, deciduous trees grow mainly in temperate and southern latitudes, while conifers grow in northern ones.

Depending on the species and climatic conditions of growth, trees have different heights and trunk diameters. However, they fall into three categories. The first includes trees of the first magnitude, which reach a height of 20 m or more. These are spruce, cedar, larch, pine, birch, aspen, linden, oak, ash, maple, etc.

In the tropics and subtropics, the height of individual trees reaches 100 m or more. The second category includes trees of the second magnitude, having a height of 10–20 m. These are, in particular, willow, alder, mountain ash, etc. The third category is trees of the third magnitude, whose height is 7–10 m. These are apple, cherry, juniper, etc. .

The diameter of the tree trunk varies mainly from 6 to 100 cm or more and depends on the species, age of the trees and climatic conditions of growth. In some cases, the diameter of a tree trunk can exceed 3 m - in oak, poplar and some other species.

Wood is obtained by cutting tree trunks after removing branches. In this case, the yield of wood is 90 or more percent of the volume of the tree trunk. At the initial stage of wood processing, a transverse, or end, section of the trunk is made.

On the cross section, the following are distinguished: the bark covering the trunk from the outside and consisting of the outer layer - the crust and the inner layer - the bast cambium - a thin layer invisible to the eye between the bark and the wood (during the growth of trees, the living cells of the cambium divide, and due to this the tree grows in thickness); sapwood - living zone of wood; the core, which is adjacent to the core of the trunk and is a dead central zone that does not participate in physiological processes; the core, located in the center and representing a loose tissue with a diameter of 2–5 mm or more (depending on the species and age of the tree).

In the timber industry in Russia, the main object of harvesting is tree trunks, and branches and branches are burned or used for firewood. In Canada, Sweden and Finland, all components of trees are recycled, so the loss of wood there is minimal, and the yield of paper, cardboard and other things is maximum.

2. Macroscopic structure of wood

With a cross section of a tree trunk, you can establish the main macroscopic features: sapwood, heartwood, annual layers, medullary rays, vessels, resin canals and medullary repetitions.

In young trees of all species, wood consists only of sapwood. Then, as they grow, the living elements around the core die off, and the moisture-conducting paths become clogged, and extractive substances gradually accumulate in them - resins, tannins, dyes. Some trees - pine, oak, apple and others -

the central zone of the trunk acquires a dark color. Such trees are called sound. In other trees, the color of the central zone and sapwood of the trunk is the same. They're called non-core.

Kernelless trees are divided into two groups: ripe-woody(linden, fir, beech, spruce), in which the humidity in the central part of the trunk is less than in the peripheral, and sapwood, which have humidity cross section the trunk is the same (birch, maple, chestnut, etc.). Moreover, the mass of sapwood decreases from the top to the butt, as well as with an increase in the age of the tree.

The age of trees can be determined by the number of annual layers that grow one per year. These layers are clearly visible on the cross section of the trunk. They are concentric layers around the core. Moreover, each annual ring consists of an inner and outer layer. The inner layer is formed in spring and early summer. It is called early wood. The outer layer is formed by the end of summer. Early wood has a lower density than late wood and is lighter in color. The width of the annual layers depends on a number of reasons: firstly, on the weather conditions during the growing season; secondly, on the growing conditions of the tree; thirdly, from the breed.

On a cross section of trees, you can see the core rays extending from the center of the trunk to the bark. In hardwoods, they occupy up to 15% of the volume of wood, in conifers - 5–6%, and the greater their number, the worse the mechanical properties of wood. The width of the core rays ranges from 0.005 to 1.0 mm, depending on the tree species. Softwood wood differs from hardwood wood in that it contains cells that produce and store resin. These cells are grouped into horizontal and vertical resin ducts. The length of the vertical passages ranges from 10–80 cm with a diameter of about 0.1 mm, and the horizontal resin passages are thinner, but there are a lot of them - up to 300 pieces per 1 cm 2.

Hardwood has vessels in the form of a system of cells for the transfer of water and minerals dissolved in it from the roots to the leaves. Vessels have the form of tubes with an average length of 10 cm and a diameter of 0.02-0.5 mm, and in trees of some species they are concentrated in the early zones of the annual layers. They are called annular.

In trees of other species, the vessels are distributed over all annual layers. These trees are called diffuse-vascular.

3. Microscopic structure of coniferous and hardwood wood

Coniferous wood has a certain microstructure, which can be established using microscopes, as well as chemical and physical research methods. Coniferous wood differs from hardwood in a relatively regular structure and simplicity. The structure of coniferous wood includes the so-called early and late tracheids.

As established by research, early tracheids function as conductors of water with minerals dissolved in it, which comes from the roots of the tree.

Tracheids are in the form of strongly elongated fibers with co-cut ends. Studies have shown that in a growing tree, only the last annual layer contains living tracheids, and the rest are dead elements.

As a result of the research, it was revealed that the core rays are formed by parenchymal cells, along which reserve nutrients and their solutions move across the trunk.

The same parenchymal cells are involved in the formation of vertical and horizontal resin ducts. Vertical resin canals in coniferous wood, found in the late zone of the annual layer, are formed by three layers of living and dead cells. Horizontal resin ducts were found in the medullary rays.

According to the research results of professor V. E. Vikhrova, pine wood has the following microscopic structure:

1) cross section;

2) radial incision;

3) tangential cut.

Rice. 1. Sections of a tree trunk: P - transverse, R - radial, T - tangential

As established by research, the microstructure of hardwood compared to coniferous wood has a more complex structure.

In hardwood, vascular and fibrous tracheids serve as conductors of water with minerals dissolved in it. The same function is performed by other vessels of wood. The mechanical function is performed by libriform fibers and fibrous tracheids. These vessels are in the form of long vertical tubes, consisting of individual cells with wide cavities and thin walls, and the vessels occupy from 12 to 55% of the total volume of hardwood. The largest part of the volume of hardwood is made up of libriform fibers as the main mechanical fabric.

Libriform fibers are elongated cells with pointed ends, narrow cavities and powerful walls with slit-like pores. Fibrous tracheids, like libriform fibers, have thick walls and small cavities. In addition, it was found that the core rays of deciduous wood unite the main part of parenchymal cells, and the volume of these rays can reach 28–32% (this figure applies to oak).

4. Chemical composition of wood

The chemical composition of wood depends partly on its condition. The wood of freshly cut trees contains a lot of water. But in a completely dry state, wood consists of organic substances, and the inorganic part is only from 0.2 to 1.7%. During the combustion of wood, the inorganic part remains in the form of ash, which contains potassium, sodium, magnesium, calcium and, in small quantities, phosphorus and other elements.

The organic part of wood of all species has approximately the same elemental composition. Absolutely dry wood contains on average 49-50% carbon, 43-44% oxygen, about 6% hydrogen and 0.1-0.3% nitrogen. Lignin, cellulose, hemicellulose, extractive substances - resin, gum, fats, tannins, pectins and others - make up the organic part of wood. Hemicellulose contains pentosans and genxosans. Coniferous species have more cellulose in the organic part, while deciduous species have more pentosans. Cellulose is the main component of the cell walls of plants, and it also provides the mechanical strength and elasticity of plant tissues. As a chemical compound, cellulose is a polyhydric alcohol. When cellulose is treated with acids, it is hydrolyzed with the formation of ethers and esters, which are used for the production of films, varnishes, plastics, etc. In addition, during the hydrolysis of cellulose, sugars are formed, from which ethyl alcohol is obtained by fermentation. Wood pulp is a valuable raw material for paper production. Another component of the organic part of wood, hemicellulose, is polysaccharides of higher plants, which are part of the cell wall. In the process of processing cellulose, lignin is obtained - an amorphous polymer substance of a yellow-brown color. The largest amount of lignin - up to 50% - is formed during the processing of coniferous wood, and its yield from hardwood is 20–30%.

Very valuable products are obtained during the pyrolysis of wood - dry distillation without air at temperatures up to 550 ° C - charcoal, liquid and gaseous products. Charcoal is used in the smelting of non-ferrous metals, in the production of electrodes, medicine, as a sorbent for sewage treatment, industrial waste, and for other purposes. Such valuable products as gasoline antioxidant, antiseptics - creosote, phenols for the production of plastics, etc. are obtained from the liquid.

In the organic part of coniferous wood there are resins that contain terpenes and resin acids. Terpenes are the main raw material for the production of turpentine. The resin secreted by the coniferous tree serves as a raw material for the production of rosin.

In the process of wood processing, extractive substances are obtained, including tannins, used for leather dressing - tanning. The main part of tannins are tannins - derivatives of polyhydric phenols, which, when processing leather, interact with their protein substances and form insoluble compounds. As a result, the skins acquire elasticity, resistance to decay and do not swell in water.

Wood(bot.). - In everyday life and technology, wood is called the inner part of the tree, lying under the bark. In botany under the name Wood, or xylem, refers to a tissue or collection of tissues formed from procambia or cambium(see this word and article); it is one of the components of the vascular fibrous bundle and is usually opposed to another component of the bundle, originating from the same procambium or cambium - bast, or phloem. During the formation of vascular-fibrous bundles from procambium, 2 cases are observed: either all procambial cells turn into elements of wood and bast - so-called. closed bundles (higher spore, monocots and some plants), or on the border between wood and bast there remains a layer of active tissue - cambium and bundles are obtained open(dicotyledons and gymnosperms). In the first case the wood remains constant and the plant is unable to thicken; in the second, thanks to the activity of the cambium, every year the amount of wood arrives, and the stem of the plant gradually thickens. In our tree species, the wood lies closer to the center (axis) of the tree, and the bast is closer to the circle (periphery). In some other plants, a different mutual arrangement of wood and bast is observed (see). The composition of wood includes already dead cellular elements with stiffened, mostly thick shells; the bast, on the contrary, is composed of living elements, with living protoplasm, cell sap, and a thin non-wooden shell. Although in the bast there are dead, thick-walled and stiff elements, and in wood, on the contrary, they are alive, but from this, however, the general rule does not change significantly. Both parts of the vascular-fibrous bundle also differ from each other in their physiological function: along the wood rises up from the soil to the leaves, so-called. raw juice, i.e. water with substances dissolved in it, but the educational one descends along the bast, otherwise plastic, juice (see. Juices in the plant). The phenomena of lignification of cells. shells are due to the impregnation of the cellulose shell with special substances, usually combined under the general name. lignin. lignin and, at the same time, the lignification of the shell is easily recognized with the help of some reactions. Due to lignification, plant shells become stronger, firmer and more resilient; however, with a slight permeability to water, they lose their ability to absorb water and swell.

Wood is composed of several elementary organs, otherwise histological elements. Following Sanio, there are 3 main groups, or systems, of elements in the wood of dicotyledonous and gymnosperms: parenchymal, luboidal and vascular. Each system has 2 types of elements, and in total there are 6 types of histological elements, and even cells of the core rays are attached as the 7th (see. Woody plants).

I. parenchymal system. It consists of 2 elements: woody(or wood) parenchyma and so on. replacement fibres. During the formation of cells of the woody parenchyma from the cambium, the cambial fibers are separated by horizontal partitions, so that a vertical row of cells is obtained from each fiber; while the end cells retain the pointed shape of the ends of the cambial fiber (see Table. Fig. 1 e- isolated by maceration of beech wood parenchyma cells; rice. 2 R- woody parenchyma cells in Ailanthus; tangential (see below) section Wood). Wood parenchyma cells are characterized by relatively thin walls; the latter are always without spiral thickening, but are provided with simple round closed pores. Reserve substances accumulate inside the cells in winter, mainly starch; but sometimes chlorophyll is also found in them, and crystals of oxalic-calcium salt. In addition, the wood parenchyma probably plays a role in the movement of water. As a constituent element of wood, it is very common; it is, however, very scarce in many conifers, and, according to Sanio, not at all in the yew (Taxus baccata). The second element of the parenchymal system is replacement fibers(E rsatzfasern) - in some cases replace a missing woody parenchyma (hence the name); in others, they are found together with elements of the latter. In structure and function, they are similar to the cells of the woody parenchyma, but are formed directly from the cambial fibers, i.e., without prior separation of the latter by transverse partitions.

II. . The two elements distinguished here are called libriform[the name is given by the similarity of the elements of this system (fibrae sive cellulae libriformes) with the fibers of thick-walled bast (liber)] simple(i.e. without baffles) and cloisonne. tic, elongated and pointed at the ends, completely closed cells of a simple libriform reach a very significant length (½ and up to 2 mm). Their stiffened walls are covered with extremely rare and small, mostly slit-like, simple or bordered pores (Fig. 1 d, rice. 2 lf). The walls are so thick that the lumen of the cell turns into a very narrow channel. In general, libriform is the thickest element of wood; it is he who predominantly or exclusively gives the tree a fortress. As for the internal cavity of the libriform cells, in most cases it is filled with air. The septate libriform differs from the simple one only in that after the final thickening of the fiber walls, the latter is partitioned off by one or several thin transverse septa into separate cells located one above the other. Sometimes such transverse partitions have pores (in grapes). The cloisonné libriform of all wood elements is the least common.

III. Vascular or tracheal system. Its composition includes present vessels (trachea) and vascular cells or fibers commonly referred to as tracheids. have the appearance of elongated (prosenchymal) spindle-shaped cells (fibers). For the most part, they are shorter and not as thick-walled as libriform cells, approaching in this respect real vessels. But in some cases, they can reach a very significant length (up to 4 mm in pine) and greatly thicken their shells. In general, tracheids are an intermediate and transitional element between a simple libriform and real vessels. distinctive and hallmark for them there are bordered pores (Fig. 3, side facing the reader; Fig. 5 b), covered with a thin median closing membrane; in the cavity of the tracheids, closed on all sides, there are water and air. According to their function, tracheids are considered water-bearing organs, but sometimes they also serve for mechanical purposes, giving strength to wood, for example. in conifers. Coniferous wood consists almost exclusively of tracheids alone, located here in regular radial rows. In each radius, cells stand approximately at the same height, which, in turn, is the result of the origin of the entire radial row from the same cambial cell. On fig. 3 such radial rows are visible in the longitudinal and transverse section, there are 8 rows in the transverse section. On fig. 4 radial rows go in the direction abc(cross section). Fringed pores are located almost exclusively on the radial walls alone (Figs. 3 and 5 b), as a result of which water in coniferous wood occurs easily in the direction of the periphery of the organ and difficult in the direction of the radius. In pine, the movement of water in the radial direction (from outside to inside and back) occurs only along the tracheids of the core rays (Fig. 5 ff- horizontally arranged tracheids of the medullary rays); in spruce, fir and larch, the movement of water along the radius and especially its inflow from the last annual layer to the cambium is greatly facilitated by the fact that in them the last tracheids of each annual layer are equipped, in addition to large pores on the radial walls, with numerous small pores on the tangential ( Fig. 3, right side). Spring tracheids are noticeably different from summer ones, and especially from autumn ones, as a result of which it is possible to distinguish coniferous wood, or rings. Rice. 4 represents a cross section of spruce timber ( abc- a layer of one year). In spring, wide thin-walled elements are formed from the cambium ( a), especially suitable for upward movement of large quantities of water. The more abundantly developed the needles of a tree and the more intense, therefore, its evaporation, the wider the belt occupied in the annual layer by wide thin-walled tracheids. As the summer progresses, the walls of the tracheids become thicker and thicker, still remaining wide, more or less isodiametric to be more precise ( b). The worse the nutritional conditions of the tree, the less such tracheids are formed, and sometimes they may be completely absent! Thus, the study of the internal structure introduces us to past conditions growth. By autumn, the diameter of the tracheids in the direction of the radius becomes smaller and smaller: a belt of autumn narrow, as if flattened elements is obtained (Fig. 4 With- cross section; rice. 5 a- longitudinal section), thick-walled with good nutrition, thin-walled - with poor nutrition. In winter, new cells are no longer formed, and with the onset of spring, the cambium gives rise to a new layer of spring, wide and thin-walled tracheids. Where the autumn elements come into contact with the spring ones, a sharply pronounced boundary of the annual layer passes in conifers (two such boundaries are visible in Fig. 4).

The structure and tracheids of deciduous trees are somewhat different than those of conifers (Fig. 1 With- isolated beech tracheid). Here, tracheids have pores on all sides, due to which the movement of water occurs equally easily both in the direction of the periphery and along the radius. Tracheids in hardwoods are mostly grouped around vessels.

True vessels (tracheae) look like long tubes. They are formed from vertical rows of cambial cells; at the same time, the cells are soldered to each other, and the transverse partitions separating them are drilled with holes. Such a composition of the vessel from individual cells-segments is especially clearly detected when the vessels are macerated: the latter break up along the partitions into separate sections (see Fig. 1 a and b).

WOOD.

The drilling of partitions occurs differently. Sometimes one large round hole is formed, and only a small narrow ring remains from the septum. Such cases are observed mainly in horizontal or only slightly inclined partitions (Fig. 1 a). At partitions located obliquely, several elliptical openings are usually formed, located one above the other: it turns out what is called a stair-perforated or simply stair partition (Fig. 1 b). Between these two extreme forms there are intermediate ones. Separate segments of the vessels are cylindrical, prismatic, sometimes barrel-shaped, moreover, of various lengths. The first vessels formed from the procambium have long segments, while the vessels formed later from the cambium, when the growth of the organs in length has already ended, are composed of much shorter segments. The length of the whole vessel can be equal to the length of the whole plant from the roots to the very leaves. Vessel walls stiffen early, but in most cases remain thin. The thickening of the longitudinal walls is always uneven, and there are several types of such thickening: annular, spiral, mesh, ladder and punctate thickening (see Plant cell). Depending on the shape of the thickening, the vessels themselves are called annular, spiral, mesh, ladder and point. Ringed and usually formed in the early life of the plant; in hardwoods - only in the first year of life, and are found only in the innermost part of the wood, in the so-called core tube, component primary wood[the earliest Wood, formed from the procambium, is called primary, the latest, arising from the cambium, is called secondary], in all recycled wood they have only punctate vessels, usually with round bordered pores (Fig. 1 a, b; rice. 2 gg). Like the length, the width of the vessels is very diverse. The first annular and spiral vessels that arose from the procambium are very narrow, at the same time, as we have seen above, their segments differ from other vessels in the greatest length; on the contrary, later punctate vessels have short segments, the width of which is sometimes so significant that they are visible on a transverse section of wood even with the naked eye, appearing as rounded pores or holes. Vessels, however, are completely absent in all secondary coniferous wood (it constitutes the main mass of the tree) - a feature that makes it easy to distinguish coniferous wood from any other. In hardwood species, the distribution of vessels among other organs of wood is different, which also often provides excellent signs for distinguishing species according to wood. Fig. 7), while in the oak the larger vessels, visible even to the naked eye, are confined to the spring part of the layer (Fig. 8), forming a spring ring of vessels (Frü hjahrsporenkreis). Such rings significantly help in distinguishing individual annual layers (Fig. 6, gg). In other plant species, the vessels are collected in peripheral wavy lines, several lines in each annual layer (in the elm, Ulmus effusa).

Vessels are dead elements. Their protoplasmic content disappears early and is replaced by an aqueous liquid alternating with rarefied air bubbles. Formerly they were mistaken for air tubes, but now they are considered as paths in the plant. In many trees and shrubs, the interior of the vessels is partially or completely filled with parenchymal cells. (filling or performing Fü llzellen or Thyllen), derived from the cells of the woody parenchyma. Cells of woody parenchyma adjacent to the vessel give sac-like processes inside into the cavity of the vessel through the pores. The processes are separated by a septum from the cells that produced them, which remained outside the vessel, grow, multiply by division, and little by little fill the cavity of the vessel. Spare starch sometimes accumulates in filling cells.

The seventh element is wood - core rays - are composed of parenchymal cells, elongated in a horizontal direction or located in a brick-like manner (Fig. 1 f[Rice. one g shows the parenchymal cells of the medullary rays in beech, somewhat deviating from the usual form.]; 6, b, With). They have the form of veinlets of various thicknesses (widths) and heights, crossing in the radial direction a mass of prosenchymal (elongated in length parallel to the axis of the plant) elements. aii) section Wood, but also on two longitudinal: radial ( aai) and tangential (dd).]. The cells that make up their composition are similar, in general, to the cells of the woody parenchyma (live, capable of accumulating starch). In many conifers, in the core rays, in addition to the parenchyma, there are also tracheids (Fig. 5 e - parenchyma, f- tracheids). Distinguish between primary and secondary rays. The primary rays stretch from the core to the primary cortex and represent the remnant of the main tissue (see Plant stem and Plant tissues), while the secondary rays are formed from the cambium and never reach either the core or the primary cortex; they are shorter than the primary rays and the shorter the later they formed from the cambium (Fig. 6 cc). Further, there are narrow (single-row) and wide (multi-row) rays. Narrow ones consist of only one radial. row of cells (Fig. 2 st[to the tangent. section]; rice. 3; rice. 6, cc), wide - from several (Fig. 6 b and d; rice. eight). The number of medullary rays, their width and height are extremely varied in different plants. In general, beams, along with vessels, provide excellent features for recognizing wood species. For oak wood, for example, broad beams are very characteristic, easily visible to the naked eye (Fig. 8). For conifers, the internal microscopic structure of the rays is characteristic; in all pines (Pinus), the parenchymal cells of the rays are bordered above and below by several rows of very typical tracheids (Fig. 5 ff), in fir, the rays consist of parenchymal cells alone; in addition, in fir all the rays are narrow and in the wood there are no resin passages, while in pine, spruce and larch there are both resin passages and both types of rays (narrow and wide). The purpose (function) of the core rays consists partly in the accumulation of reserve substances, and partly in the conduction of juices and water in a horizontal direction. Usually, only some of the first 6 elements described above are included in the composition of wood; but they are combined with each other in quite different ways. and elements were especially carefully studied by Sanio. He compiled a special table, guided by which one can identify a plant from a small piece of wood (see literature). As mentioned above, in dicots and gymnosperms, the amount of wood increases from year to year due to the formation of new annual layers from the cambium. The shape and width of such layers are not the same in different plants, and even in the same plant they can vary depending on many conditions, both internal (age, for example) and external (climate, soil, etc.; see Woody plants). In addition, in the same tree, layers of different ages can differ significantly from each other both in shape and histological structure, and in chemical composition. Spirals are also found in trees, for example, only in the first, innermost and, at the same time, the oldest annual layer, which includes primary Wood (see above). In physicochemical terms, all layers can be similar, or the inner ones differ from the outer ones, and the wood is separated into the inner part, or core (Kernholz, duramen), and the outer, or sapwood (Splint, alburnum - see Core and). Heartwood is heavier, harder, stronger than sapwood, in addition, it differs from the latter in most cases in a darker color. This color is brown in oak, dark brown in cherry, reddish in larch; in some tropical plants, the colors are even sharper: red in mahogany (Caesalpinia echinata), blue in logwood (Haemotoxylon campechianum), black in black, or ebony, wood (Diospyros Ebenum). During the transformation of sapwood into a heartwood, it is mainly the chemical composition of the wood that changes, and not its histological structure. Various substances accumulate in cavities and especially in cell membranes: resins, wood gums, tannins, and sometimes dyes, some of which are used in practice (see). Physiologically, the core differs from the rest of the wood in its negative, so to speak, dead properties: it is not capable of periodically accumulating starch and other reserve substances, it is not even capable of conducting water.

Literature. Sanio, "Vergleichende Untersuchungen über die Elementarorgane des Holzkö rpers" and "Vergleichende Untersuchungen über die Zusammensetzgung des Holzkörpers" ("Botanisch e Zeitung", 1863); De Bari, "Comparative anatomy of the vegetative organs of phanerogamous and figurative plants" (translated by prof. A. N. a, issue I-II, St. Petersburg, 1877-80); Haberlandt, "Physiologische Pflanzenanatomie" (1884); , "Brief practical course Plant Histology for Beginners" (translated by S. a, 1886); Strasburger, "Das botanische Practicum" (1887), Prof., "Course in Plant Anatomy" (1888); Tschirch, "Angewandte Pflanzenanatomie" (1889); Robert Hartig, "Die anatomisch en Unterscheidungsmerkmale der wichtigeren in Deutschland wachsenden Hö lzer" (1890, 3rd ed.) and "Lehrbuch der Anatomie und Physiologie der Pflanzen" (1891); Van-Tieghem, "Trait é de Botanique" (vol. I , 1891); and Yashnov, "Identification of wood, seeds and branches according to tables" (1893). Special literature is indicated in the above-mentioned works.