The Oak (Ward)/Chapter III
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The Seedling and Young Plant
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the seedling and young plant.
Before proceeding to describe the further growth and development of the seedling, it will be well to examine its structure in this comparatively simple stage, in order to obtain points of view for our studies at a later period. For many reasons it is advantageous to begin with the root-system. If we cut a neat section accurately transverse to the long axis of the root, and a few millimetres behind its tip, the following parts may be discerned with the aid of a good lens, or a microscope, on the flat face of the almost colorless section. A circular area of grayish cells occupies the centre—this is called the axis cylinder of the young root (Fig. 5, a, a). Surrounding this is a wide margin of larger cells, forming a sort of sheathing cylinder to this axial one, and termed the root-cortex. The superficial layer of cells of this root-cortex has been distinguished as a special tissue, like an epidermis, and as it is the layer which alone produces the root-hairs, we may conveniently regard it as worthy of distinction as the piliferous layer (Fig. 5, e).
Similar thin sections a little nearer the tip of the root would show a more or less loose sheath of cells in addition to and outside this piliferous layer. This is the root-cap, which is a thimble-shaped sheath of looser cells covering the tip of the root as a thimble covers the
Fig. 5.—A. Transverse section of young root under a lens, showing the axis cylinder, a; epidermis or piliferous layer, e; and the cortex between. B. The same, more highly magnified: c, cortex; p, phloëm ; x, xylem; C. A portion still more highly magnified: ph, phloëm; p, pith; per, pericycle; sh, sheath (endodermis); other letters as before.
end of the finger, only we must imagine the extreme tip of the finger organically connected with the inside of the cap to make the analogy suitable (see Fig. 6). The rest of the section would be much as before, excepting that the distinction between the axial cylinder and the root-cortex would be less marked.
Now contrast a section cut a couple of inches or so away from the tip, in the region where the root-hairs are well developed. Here we find the axial cylinder much more strongly marked than before, and the piliferous layer is very clearly distinguished by the fact that it gives off the root-hairs, each hair arising from one of its cells.
A little investigation shows that the axial cylinder is thus strongly marked because certain dark-looking structures have now been formed just inside its boundary—i. e, just inside the line which delimits it from the root-cortex. These dark structures are the sections of several fine cords or bundles, called vascular bundles, which can here be traced up and down in the root. As the section shows, these bundles are arranged at approximately equal distances in a cylinder; they form the vascular system of the root, and they always run along the region just inside the outer boundary of the axial cylinder (Fig. 5, b, p and x).
If we compare our successive transverse sections, and cut others at various levels along the young root, it will be clear that, as we pass from the tip of the root to parts farther behind, certain changes must be going on, which result first in the definite marking out of the axial cylinder, and then in the development of these vascular bundles and of other parts we will not describe in detail. If, in addition to these successive transverse sections, we examine a carefully prepared longitudinal section, cut
Fig. 6.—Diagrammatic section through the end of the root of the oak. c, root-cortex; e, piliferous layer; rc, root-cap; m, the true embryonic tissue (so-called "growing-point"); ph, phloëm; x, xylem. so as to pass accurately through the median plane of the root, the comparison not only establishes the above conclusion, but it enables us to be certain of yet other facts (Fig. 6). Such a section shows the root-cap covering the tip as a thimble the end of the finger, and the rim of this root-cap is evidently fraying away behind; the cells of which it is composed die and slough off as the root pushes its way between the abrading particles of soil. Obviously this loss of worn-out tissue must be made good in some way, and closer examination shows how this occurs. The extreme tip of the root proper fits closely into the cap, and evidently adds cells to the inside of the latter, and thus replaces the old ones which are worn away. At this true tip of the root, moreover, we make another discovery, namely, that all the cells are there alike in shape, size, and other peculiarities; and if we could take a transverse section exactly at this place we should see no differentiation into axial cylinder and root-cortex, etc.; the small circular mass would consist of cells all alike, and with very thin walls and full of dense protoplasm. This undifferentiated formative tissue is called the embryonic tissue of the root (Fig. 6, m). A little behind this we see the axis-cylinder and root-cortex already formed; still farther away we see the vascular bundles appearing, first as very thin cords, and then getting stronger and stronger as we recede from the tip (Fig. 6, ph and x); and similarly we trace the gradual development of the other parts in acropetal succession—i.e., the nearer we go to the apex the younger the parts are.
Now, there is a conclusion of some importance to be drawn from the putting together of these facts—namely, that all the structures found between the embryonic tissue at the tip of the root and the place where the root joins the stem have been gradually formed from the embryonic tissue in acropetal succession. We may picture this by marking a given level on the root, some distance away from the tip, where the axis-cylinder is sharply marked and has well-developed vascular bundles, the root-cortex is distinct, and the piliferous layer bears root-hairs, and remembering that so many days or weeks ago this very spot was in the then growing-point, and consisted of embryonic tissue with the cells all alike. Or we may put it in a different way thus: the present growing-point consists of embryonic cells all alike; in a few days some of these cells will have changed into constituents of the axis-cylinder and cortex, and subsequently some of them will give rise to vascular bundles, etc. Not all, however; and it is necessary to understand that as the embryonic tissue moves onward and leaves the structures referred to in its wake, it does so by producing new embryonic cells in front—i.e., between the present ones and the root-cap.
We must now look a little more closely into the structure of the axial cylinder, at a level a little behind the region where the root-hairs are produced on the piliferous layer.
A thin transverse section in this region shows that the root-hairs have all died away, and the walls of the cells of the piliferous layer are becoming discolored, being, in fact, converted into a brown, cork-like substance impervious to moisture, or nearly so; consequently the piliferous layer is no longer absorptive, and it will soon be thrown off, as we shall see.
The cortex offers little to notice, except that its cells are being passively stretched or compressed by the growth and processes going on in the axial cylinder; and it is this cylinder that attracts our special attention, and several points not noticed before must now be examined in some detail.
In the first place, the cylinder is demarkated off from the cortex by a single layer of cells shaped like bricks, and with a sort of black dot on the radial walls; this is called the endodermis, and may be regarded as a sheath limiting what belongs to the axis-cylinder (Fig. 5, c, sh). Inside this endodermis are about two rows of thin-walled cells full of protoplasm, and forming a continuous layer beneath the endodermis. This layer is termed the pericycle (Fig, 5, c, per) and it is a very important structure, because its cells give rise, by repeated divisions, to the lateral rootlets, which then grow out and burst their way through the endodermis, cortex, and piliferous layer, and so reach the soil. It is, of course, necessary to bear in mind that the endodermis and pericycle are concentric cylinders superposed on the axis of the root, as it were, and only appear as rings on the transverse section.
Inside the pericycle are arranged the vascular bundles, and we shall have to devote a few words of explanation to these remarkable and somewhat complex structures.
The section shows that there are about ten alternating groups of tissue constituting these bundles, and again the reader must bear in mind that each group is the transverse section of a long cord running up and down the root. Of these groups five are much more conspicuous than the other five, because they consist chiefly of more or less polygonal openings with firm, dark contours. These are the xylem vessels of the vascular bundles (Fig. 5, c, x), and we must note the following facts about them: In the first place, they are smaller nearer the pericycle than they are nearer the center of the axial cylinder, and the comparison of numerous transverse sections at different levels of the root would prove that the smallest vessels are the first to develop; whence we learn two facts—namely, that the xylem vessels of the young root are developed in centripetal order, and that the later ones have a larger caliber than those formed earlier.
If longitudinal sections are compared with these transverse ones—and I may here observe that it is only by means of numerous such comparisons that these matters have been gradually discovered—it is found that each vessel is a long tube, usually containing air and water when complete, the lateral walls of which are curiously and beautifully marked with characteristic thick and thin ornamentation. It must suffice here to say that the small, outer, first-formed vessels are marked with a spiral thickening, reminding one of caoutchouc gas-tubing kept open by means of a spiral wire inside; while the larger ones, developed later, usually have numerous small pits on their walls, reminding one of mouths, and the structure of which is very curious. Consequently these groups of xylem vessels are said to consist of spiral and pitted vessels, and their chief function is to convey water up the root to the stem (cf. Fig. 16). Packed in between these vessels are certain cells known as the wood-cells.
Returning to the transverse section, we saw that between each xylem group described above there is a group of structures differing from the latter in their less distinct outlines; these alternate groups are known as phloëm, and we may shortly examine the elements of which they are composed, as before, by comparing sections of various kinds.
Here, again, we find the chief structures in the phloëm are also vessels—i.e., long, tubular organs—but very different in detail from the vessels of the xylem.
In the first place, their walls are thin and soft, and composed of the unaltered cellulose which is so characteristic of young cells (instead of being hard, like the lignified wails of the xylem vessels); then, again, they contain protoplasm and other organized cell contents, instead of merely air and water. Finally, they are not so completely tubular as the typical xylem vessels are, because the transverse septa of the constituent cells are not absorbed, but are merely pierced by fine strands of protoplasm, and therefore look like sieves when viewed from above—whence the name "sieve-tubes." In the phloëm also we find cells—phloëm-cells—packed in between the sieve-tubes.
If we shortly summarize the above we find that the root consists of an axis-cylinder surrounded by a cortex and the piliferous layer. At the tip the whole is covered by the root-cap, which is organically connected with the embryonic tissue which forms all these structures. The axis-cylinder is somewhat complex; it is sheathed by the endodermis and the pericyle, the latter of which gives origin to the new rootlets. Inside the pericycle are the vascular bundles running up and down as separate, alternate cords of xylem and phloëm;
Fig. 7.—Portion of young growing ends of more advanced root, with numerous rootlets. Some of the latter are much branched into tuft-like collections, m; these form the so-called Mycorhiza. Natural size.
the xylem consists of vessels and cells, the former developed centripetally, while the phloëm consists of sieve-tubes and cells. Any cell-tissue which may lie in the center of the axial cylinder, and surrounded by the vascular bundles, corresponds, in popular language, to pith; any that runs between the bundles corresponds to medullary rays.
We now turn to the root as a whole, and examine its behavior in the soil as the young seedling develops further, and in the light of the above anatomical facts.
Although the root-system of the young plant is regularly constituted of a series of lateral rootlets springing from the primary root, the orderly arrangement is soon disturbed when the tertiary and other rootlets begin to develop from the secondary rootlets; moreover, as the age of the tree increases, the tendency to irregularity is increased owing to the production of rootlets of the higher orders at different places, thus interfering with the acropetal succession of the younger rootlets.
At first the root-system is especially engaged in boring into the soil, and, provided the latter is sufficiently deep and otherwise suitable, the tap-root will go down a foot or more in the first year. As the roots thicken they exhibit considerable plasticity, as is especially evinced on rocky ground, where the older roots may often be found in cracks in the rocks, so compressed that they form mere flattened sheets many times broader than they are thick (Fig. 8).
Fig. 8.—Portion of an older root of an oak, which had penetrated while young between two pieces of hard rock, and had to adapt its form accordingly as it thickened. (After Döbner.) It has already been mentioned that the tip of the young primary root circumnutates, and Darwin also found that the tip of the radicle is extremely sensitive to the irritation of small bodies in contact with it. It is also positively geotropic, directing itself vertically downward if the partially grown radicle is laid horizontally; and it may be assumed from the behavior of other plants of the same kind that the tip of the radicle is negatively heliotropic—i.e., it turns away from the source of light. Whether it is also sensitive to differences in the degree of moisture on different sides (hydrotropic), or to differences of temperature (thermotropic), is not known, but it may be inferred that such is the case; nor do we know whether it is affected by electric currents in the earth The root of the oak, speaking generally, is a typical root in the following respects: It consists, as we have seen, of a primary or tap root which develops secondary or lateral roots in acropetal succession, and these in their turn produce rootlets of a higher order. These secondary, tertiary, etc., rootlets arise endogenously, taking origin from the pericycle at the periphery of the strand of vascular bundles which traverse the central axis, and then bursting through the cortex to the exterior. The primary root, as well as the rootlets of all orders, are provided with a root-cap at the tips, and they all agree in being devoid of chlorophyll or stomata. From the outer layer of cells—the piliferous layer, corresponding to an epidermis—root-hairs are developed at some little distance behind the root-cap, and these superficial cellular outgrowths also rise in acropetal succession, the older ones behind dying oS as the younger ones arise farther forward. If we bear in mind all that has been shortly stated above, it will be very easy to figure the behavior of the root-system as it penetrates the ground, and the following short description of the biology of the root may render the matter clear. When the radicle commences to bore down into the soil it puts forth a large number of root-hairs from the parts a few millimetres behind the tip, and these attach themselves to the particles of soil and supply points of resistance; the tip of the radicle is protected by the slippery root-cap, and it must be borne in mind that the embryonic tissue of the growing-point consists of thin-walled cells full of relatively stiff protoplasm with very little water. Hence the growing-point is a firm body. The most active growth of the root takes place at a region several millimetres behind the root-cap, between it and the fixed point above referred to; hence the apex of the root is really driven into the ground between the particles of rock, etc., of which the latter is composed. This driving in is aided by the negative heliotropism, the positive geotropism, the circumnutation, and other irritabilities of the apical portions of the root, and it bores its way several centimetres downward. As it lengthens—by the addition of cells produced by the division of those of the embryonic tissue, and by their successive elongation—the older parts behind go on producing root-hairs, and thus a vertical cylinder of soil around the primary root is gradually laid under contribution for water containing dissolved salts, etc. In those parts of the root which are behind the growing region no further elongation occurs; hence the tips of the lateral rootlets (which have been developing in the pericycle at the circumference of the axial cylinder of vascular bundles) can now safely break through the cortex and extend themselves in the same manner from the parent root as a fixed base, without danger of being broken off by the elongation of the growing parts. Each of these secondary rootlets grows out at an obtuse angle from the primary root, and not vertically downward, and as it does so a similar wave of root-hairs is developed along it; thus a series of nearly horizontal radiating cylinders of soil are placed under contribution as before. Then the secondary rootlets emit tertiary rootlets in all directions—these and the rootlets of a higher order growing without any particular reference to the direction of gravitation, light, etc.—and so place successive cylinders of soil in all directions under contribution as before. By this time, however, the symmetry of the root-system is being disturbed because some of the rootlets meet with stones or other obstacles, others get dried up or frozen, or gnawed off or otherwise injured, and the varying directions in which new growths start and in which the resistances are least, influence the very various shapes of the tangled mass of roots now permeating the soil in all directions.
These roots supply the ever-increasing needs for water of the shoot-system, the leaf-surface of which is becoming larger and larger, and as the greater volume of water from the gathering rootlets has all to enter the stem via the upper part of the main root, we are not surprised to find that the latter thickens, as does the stem; and so with all the older roots—they no longer act as absorbing roots, but become merely larger and larger channels for water, and girder-like supporting organs.