The Oak (Ward)/Chapter VII
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The Tree―Its Root System
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the tree—its root-system.
We may now suppose the young oak-plant to be rapidly developing into a tree. Technically the seedling is said to be a plant after the first year, and when it reaches the height of a few feet the young tree is called a sapling; these ideas are by no means well defined, however, and we may regard them as arbitrary terms of little or no scientific value.
The principal changes which are noticeable as the little tree grows larger are the gradual increase in the length and thickness of the stem, and in the number and spread of the branches put forth year after year. Corresponding with these increments, each spring sees a greater number of leaves than the one before, and it is easy to prove that the roots also become more numerous and complex each season.
The above simply expresses certain facts of observation, but it is more accurate to link them together as follows:
In each successive season of growth the young oak develops more leaves than it did before—in other words, the total area of the leaf-surface exposed to the air and sunlight is larger each successive summer than it was the previous one. Several very important consequences follow from this. In the first place, the larger area of leaf-surface evaporates more water than before, and as this water is derived from the soil the absorbing surface of the roots has to increase, or the larger supplies needed could not be obtained. In the second place, these larger and larger quantities of water require corresponding increase in the sectional area of the pipes or water conduits—i.e., the vessels of the wood—through which they have to pass in order to reach the leaves. This is insured by the increase in diameter of the stem and main root and their chief branches, a larger number of vessels, etc., being added each season. In the third place, as the leaf-crown enlarges its weight increases, and the surface it exposes to the swaying action of the wind is correspondingly greater; consequently the necessity arises for more strength and rigidity in the supporting stem, and for a larger hold on the soil on the part of the root-system, which has to withstand the lever action of the swaying tree. These needs, again, are met by the thickening of the woody parts of the shoot-axis and roots, and by the greater spread and increased number of points of contact in the soil of the latter.
Correlated with these phenomena we have the increased leaf-surface playing the part of an enlarging manufactory, which turns out increased supplies of constructive materials each summer; for it is in the leaves that the substances for making new roots and shoots, new wood, and new leaves, etc., are constructed. It is in the increased area of this leaf laboratory that the larger supplies of salts, dissolved in the larger quantities of water from the soil, are brought into relations with the increased quantities of carbonaceous substance obtained from the air in the chlorophyll corpuscles, and consequently a larger yield of plant-forming materials is possible to meet the demands of the ever-growing organs.
My present purpose is to describe how the thickening process occurs in the older roots, for it is evident at a glance that the strong woody roots of a large tree have undergone many changes since they were the thin filiform rootlets we met with in the young plant (see Fig.7). Not only have they increased in diameter, but they now consist almost entirely of wood, protected by a relatively thin, brown, corky covering, reminding one of certain kinds of bark.
The first changes which take place when the young, thin roots begin to thicken are—first the piliferous layer dies away and the outer cells of the cortex turn brown; then a cylindrical layer of cork is developed in the pericycle, and as this cork is impervious to water it cuts off the cortex from communication with the axis-cylinder, and consequently the cortex gradually shrivels up and is thrown off.
Meanwhile active divisions have been going on in the cells immediately inside the phloëm groups of the axis-cylinder (see Fig. 5), and especially by means of tangential walls. The result of this activity is the development of a cambium layer, as it is called, immediately inside the five phloëm groups of the axis-cylinder, and this layer becomes continuous all round the axis-cylinder, but is so arranged that it runs outside the primary xylem groups and inside the primary phloëm groups (Fig. 24, cam). This cambium layer is a hollow cylindrical layer of thin-walled cells, full of protoplasm, and somewhat longer than they are broad or deep, and these cells have the peculiarity of dividing very rapidly, especially by tangential walls, so that cell multiplication goes on very rapidly, and the layer would soon become very thick if no other changes occurred. As the new cells are formed, however, those on the outer side of the cylinder—i.e., those nearest the phloëm—become for the most part converted into sieve-tubes and cells of the phloëm; while the much more numerous cells formed on the inner side—i.e., nearest the center of the axis-cylinder—are chiefly converted into vessels and cells of the xylem. This xylem and phloëm developed by the cambium are termed secondary xylem and secondary phloëm respectively, and it will be noticed that whereas the secondary phloëm is deposited radially on the inner side of the primary phloëm, the secondary xylem is placed between the primary xylem groups, and not radially outside them (Fig. 24, se.x and se.ph). Moreover, the youngest vessels are now nearest the cambium, whence the order of development has become the converse of that of the primary xylem; there are also no spiral vessels formed now. In fact, the structure of the vascular bundles of the root has now changed its character, and from this point forward the root increases in thickness exactly as the stem does, whence I refer the reader to the following chapter for further details.
The development of the layers of cork which now surround the thickening axis-cylinder go on forming year after year, as the cambium forms more xylem and phloëm and so thickens the root; were this not the case, the layer of cork would soon be ruptured as the root increases in diameter. Such rupture, in fact, does occur, but the cork-forming tissue in the pericycle goes on growing and acts as a cork-cambium, and repeatedly develops more cork to make good the layers which are being split and worn off in the soil.
From what has been said it will be understood that a transverse section of an old root differs entirely in structure from that of a young one, although all the changes in the former can be correlated with the primary structures of the latter. In the first place, such a section shows no piliferous layer or cortex, both having been sloughed off long ago; the protective function of these layers is now assumed by the cork jacket (often called periderm) developed by the cork-cambium cylinder in the pericycle, and even this will not show all the cork that the cambium has developed, because many outer layers will have flaked away, just as the present outer layers are doing.
Then, inside this periderm we shall find the phloëm forming an almost continuous ring (Fig. 24, se.ph), and
Fig. 24.—Transverse sections (semi-diagrammatic) of roots of oak, to be compared with Fig. 7. The smaller figure, above, shows the cambium ring, cam, now developed as a continuous layer running inside the primary phloëm, pr.ph, and outside the primary xylem, pr.x; and the larger figure shows the results of its activity in the formation of secondary phloëm, se.ph, inside the primary, and secondary xylem, se.x, between the primary xylem groups. In both cases, ep., piliferous layer; c, cortex; P, pith; sh, endodermis. Within the latter lies the pericycle, in which the cork cambium, c.cam, is now developed.
consisting chiefly of the sieve-tubes and cells developed from the cambium cylinder, the small primary phloëm masses being almost undistinguishably pressed into (pr.ph).
In the center of the section will be a small speck, around which the microscopic primary xylem groups (pr.x) are arranged; but these, again, are merged between the relatively huge masses of secondary xylem which makes up by far the major part of the whole (se.x). The thin cambium ring can be distinguished running between the xylem and phloëm as a fine line. Certain concentric annular lines may be seen on the section, and each of these marks the position in which the cambium rested during the winter of some previous year. They are the boundaries of concentric zones, termed annual rings, and the thickness of wood which makes up any one annual ring represents the activity of the cambium during that particular year.
Traversing these annual rings at right angles are fine medullary rays. About five broader ones may be found corresponding to the radii on which the primary xylem groups were formed, but these are not developed by the cambium as the finer ones are. As I shall have to speak of annual rings and secondary medullary rays at greater length when describing the thickening processes in the stem, and as they are formed in the same way in both cases, we may defer their consideration for the present.
Mention must now be made of a remarkable biological phenomenon in connection with the roots of the oak. This is the very common occurrence of young rootlets clothed by a fungus mycelium; the mycelium is found as a thin sheet of closely-woven hyphæ continuous over the whole of the tip, and sending processes in between the cells of the dermatogen, but not into the cavities of the cells nor deeper into the tissues. Loose hyphæ also
Fig. 25.—Longitudinal section of the tip of one of the roots marked m in Fig. 7, the outer layers of which are infested with fungus hyphæ, f (mycorhiza); r.c, root-cap; m, embryonic tissue from which all originates; P, pith; sp, spiral vessels of the primary xylem; c, cortex.
radiate into the soil around, and often simulate the root-hairs of other plants, which, in fact, they are said to replace (Fig. 25, f). These hyphæ are extremely fine tubes of a cellulose-like substance, filled with the living protoplasms of the fungus, and possess the remarkable property of being able to bore their way through or between the cellulose walls of the roots. The fungus attacks the plant about the second year, and it is not difficult to find true root-hairs on the young root-system when the apices are still free from the fungus mycelium. The parts of the root attacked alter their form slightly; they grow more slowly in length, and assume a fleshy, coral-like appearance (Fig. 7, m). Such a fungus-clothed root is called a mycorhiza, and the view is gaining ground that the symbiosis between the fungus and the root is of advantage to the oak. It has even been suggested that the mycelium performs the functions of root-hairs to the root, absorbing water and nutritive materials from the soil and passing them on to the oak, in return for a certain small proportion of organic substance which the latter can well afford. At any rate, it may be that the fungus hurries the decomposition of vegetable remains in such a way that they become available to the root sooner than would otherwise be the case. The systematic position of these remarkable fungi is not yet ascertained, but there is some evidence for the view that the mycelium is that of a truffle, though the question is still an open one.