Part 4 (1/2)
In a transverse section of a conifer, for example Douglas spruce, Fig.
8, the wood is seen to lie in concentric rings, the outer part of the ring being darker in color than the inner part. In reality each of these rings is a section of an irregular hollow cone, each cone enveloping its inner neighbor. Each cone ordinarily const.i.tutes a year's growth, and therefore there is a greater number of them at the base of a tree than higher up. These cones vary greatly in _thickness_, or, looking at a cross-section, the rings vary in _width_; in general, those at the center being thicker than those toward the bark. Variations from year to year may also be noticed, showing that the tree was well nourished one year and poorly nourished another year. Rings, however, do not always indicate a year's growth.
”False rings” are sometimes formed by a cessation in the growth due to drouth, fire or other accident, followed by renewed growth the same season.
[Ill.u.s.tration: Fig. 8. Section of Douglas Fir, Showing Annual Rings and Knots at Center of Trunk. _American Museum of Natural History, N.
Y._]
In a radial section of a log, Fig. 8, these ”rings” appear as a series of parallel lines and if one could examine a long enough log these lines would converge, as would the cut edges in a nest of cones, if they were cut up thru the center, as in Fig. 9.
[Ill.u.s.tration: Fig. 9. Diagram of Radial Section of Log (exaggerated) Showing Annual Cones of Growth.]
In a tangential section, the lines appear as broad bands, and since almost no tree grows perfectly straight, these lines are wavy, and give the characteristic pleasing ”grain” of wood. Fig. 27, p. 35. The annual rings can sometimes be discerned in the bark as well as in the wood, as in corks, which are made of the outer bark of the cork oak, a product of southern Europe and northern Africa. Fig. 10.
[Ill.u.s.tration: Fig. 10. Annual Rings in Bark (cork).]
The growth of the wood of exogenous trees takes place thru the ability, already noted, of protoplasmic cells to divide. The cambium cells, which have very thin walls, are rectangular in shape, broader tangentially than radially, and tapering above and below to a chisel edge, Fig. 11. After they have grown somewhat radially, part.i.tion walls form across them in the longitudinal, tangential direction, so that in place of one initial cell, there are two daughter cells radially disposed. Each of these small cells grows and re-divides, as in Fig. 12. Finally the innermost cell ceases to divide, and uses its protoplasm to become thick and hard wood. In like manner the outermost cambium cell becomes bast, while the cells between them continue to grow and divide, and so the process goes on. In nearly all stems, there is much more abundant formation of wood than of bast cells. In other words, more cambium cells turn to wood than to bast.
[Ill.u.s.tration: Fig. 11. Diagram Showing Grain of Spruce Highly Magnified. PR, pith rays; BP, bordered pits; Sp W, spring wood; SW, summer wood; CC, overlapping of chisel shaped ends.]
[Ill.u.s.tration: Fig. 12. Diagram Showing the Mode of Division of the Cambium Cells. The cambium cell is shaded to distinguish it from the cells derived from it. Note in the last division at the right that the inner daughter cell becomes the cambium cell while the outer cell develops into a bast cell. _From Curtis: Nature and Development of Plants._]
In the spring when there is comparatively little light and heat, when the roots and leaves are inactive and feeble, and when the bark, split by winter, does not bind very tightly, the inner cambium cells produce radially wide wood cells with relatively thin walls. These const.i.tute the spring wood. But in summer the jacket of bark binds tightly, there is plenty of heat and light, and the leaves and roots are very active, so that the cambium cells produce thicker walled cells, called summer wood. During the winter the trees rest, and no development takes place until spring, when the large thin-walled cells are formed again, making a sharp contrast with those formed at the end of the previous season.
It is only at the tips of the branches that the cambium cells grow much in length; so that if a nail were driven into a tree twenty years old at, say, four feet from the ground, it would still be four feet from the ground one hundred years later.
Looking once more at the cross-section, say, of spruce, the inner portion of each ring is lighter in color and softer in texture than the outer portion. On a radial or tangential section, one's finger nail can easily indent the inner portion of the ring, tho the outer dark part of the ring may be very hard. The inner, light, soft portion of the ring is the part that grows in the spring and early summer, and is called the ”spring wood” while the part that grows later in the season is called ”summer wood.” As the summer wood is hard and heavy, it largely determines the strength and weight of the wood, so that as a rule, the greater the proportion of the summer growth, the better the wood. This can be controlled to some extent by proper forestry methods, as is done in European larch forests, by ”underplanting” them with beech.
In a normal tree, the summer growth forms a greater proportion of the wood formed during the period of thriftiest growth, so that in neither youth nor old age, is there so great a proportion of summer wood as in middle age.
It will help to make clear the general structure of wood if one imagines the trunk of a tree to consist of a bundle of rubber tubes crushed together, so that they a.s.sume angular shapes and have no s.p.a.ces between them. If the tubes are laid in concentric layers, first a layer which has thin walls, then successive layers having thicker and thicker walls, then suddenly a layer of thin-walled tubes and increasing again to thick-walled ones and so on, such an arrangement would represent the successive annual ”rings” of conifers.
_The medullary rays._ While most of the elements in wood run longitudinally in the log, it is also to be noted that running at right angles to these and radially to the log, are other groups of cells called pith rays or medullary rays (Latin, _medulla_, which means pith). These are the large ”silver flakes” to be seen in quartered oak, which give it its beautiful and distinctive grain, Fig.
32, p. 38. They appear as long, grayish lines on a cross-section, as broad, s.h.i.+ning bands on the radial section, and as short, thick lines tapering at each end on the tangential section. In other words, they are like flat, rectangular plates standing on edge and radiating lengthwise from the center of the tree. They vary greatly in size in different woods. In sycamore they are very prominent, Fig. 13. In oak they are often several hundred cells wide (_i.e._, up and down in the tree). This may amount to an inch or two. They are often twenty cells thick, tapering to one cell at the edge. In oak very many are also small, even microscopic. But in the conifers and also in some of the broad-leaved trees, altho they can be discerned with the naked eye on a split radial surface, still they are all very small. In pine there are some 15,000 of them to a square inch of a tangential section. They are to be found in all exogens. In a cross-section, say of oak, Fig.
14, it can readily be seen that some pith rays begin at the center of the tree and some farther out. Those that start from the pith are formed the first year and are called primary pith rays, while those that begin in a subsequent year, starting at the cambium of that year, are called secondary rays.
[Ill.u.s.tration: Fig. 13. Tangential Section of Sycamore, Magnified 37 Diameters. Note the large size of the pith rays, A, A (end view).]
The function of the pith rays is twofold. (1) They transfer formative material from one part of a stem to another, communicating with both wood and bark by means of the simple and bordered pits in them, and (2) they bind the trunk together from pith to bark. On the other hand their presence makes it easier for the wood to split radially.
The substance of which they are composed is ”parenchyma” (Greek, _beside_, to _pour_), which also const.i.tutes the pith, the rays forming a sort of connecting link between the first and last growth of the tree, as the cambium cells form new wood each year.
[Ill.u.s.tration: Fig. 14 Cross-section of White Oak. The Radiating White Lines are the Pith Rays.]
If a cambium cell is opposite to a pith ray, it divides crosswise (transversely) into eight or ten cells one above another, which stretch out radially, retaining their protoplasm, and so continue the pith ray. As the tree grows larger, new, or secondary medullary rays start from the cambium then active, so that every year new rays are formed both thinner and shorter than the primary rays, Fig. 14.
Now suppose that laid among the ordinary thin-walled tubes were quite large tubes, so that one could tell the ”ring” not only by the thin walls but by the presence of large tubes. That would represent the ring-porous woods, and the large tubes would be called vessels, or _trache_. Suppose again that these large tubes were scattered in disorder thru the layers. This arrangement would represent the diffuse-porous woods.
By holding up to the light, thin cross-sections of spruce or pine, Fig. 15, oak or ash, Fig. 16, and ba.s.s or maple, Fig. 17, these three quite distinct arrangements in the structure may be distinguished.
This fact has led to the cla.s.sification of woods according to the presence and distribution of ”pores,” or as they are technically called, ”vessels” or ”tracheae.” By this cla.s.sification we have: