The Oak (Ward)/Chapter IV
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The Seedling and Young Plant (continued). Its Shoot-System
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the seedling and young plant (continued).
Its Shoot-system—Distribution of the Tissues.
I now proceed to describe the chief features of importance in the structure of the shoot of the young oak-plant, premising that many of the remarks may here be curtailed in view of the facts already learned in connection with the root. The first object will be to bring out the differences in the shoot as contrasted with the root, and first we may examine the structure by means of transverse sections as before. The shoot consists of all the structures developed from the plumule.
Such sections show that we have here also various definitely grouped tissues, of which we may conveniently distinguish three systems. A series of vascular bundles grouped in a close ring constitutes one of these systems; another is represented by a single layer of cells at the periphery of the section, and this is called the epidermis; and the remainder of the section composes the third system, often termed the fundamental tissue, and divided arbitrarily into three regions—the pith, the cortex, and the primary medullary rays (Fig. 9). The chief points of difference from the root are that the xylem and phloëm of these vascular bundles of the stem do not alternate on the section, as they did in the root, but the phloëm of each bundle is on the same radius as the xylem; and that there is no pericycle, for branches
Fig. 9.—Transverse sections through very young twigs of oak, showing the vascular bundles of the stem (P and X), arranged in a ring round the pith, and joined by the cambium ring—the fine line passing through the bundles; M and s, the vascular bundles passing down from the leaves—M the median bundles and s the lateral bundles. The external outline is the epidermis; the letters P P stand in the primary cortex; the letters X X stand in the pith; the primary medullary rays separate the bundles. (After Müller.)
are not developed endogenously as rootlets are. Then there are some important differences in the mode of origin of these vascular bundles in space. We saw that in the root the first-formed spiral vessels are developed at the outer parts of the axis-cylinder, nearest the cortex, and the succeeding vessels are formed in centripetal order from these points. In the young stem the exact converse occurs—the first spiral vessels arise near the center of the stem, and development proceeds centrifugally from the first. We may begin our study of the shoot by tracing the course of the vascular bundles, which, it must be remembered, are the channels of communication between the water-supply at the roots below and the leaves and young parts of the shoot above.
If we cut a transverse section of the terminal bud of the oak, as close to the tip as possible, we shall obtain a preparation of the young axis consisting entirely of embryonic tissue, all the cells of which are practically alike—small, polygonal, thin-walled cells, with large nuclei and much protoplasm, but without sap-vacuoles; these cells are in a state of active division, those in the interior dividing successively in all planes. Those which form the peripheral layer, however, are already distinguished by only dividing in the two planes at right angles to the periphery, and they constitute the primitive epidermis. There is no structure corresponding to a root-cap.
Transverse sections a little lower down show differences of the following nature: In the first place, the outline of the section tends to be somewhat pentagonal, the points of origin of the very young leaves being at the angles of the pentagon in accordance with their phyllotaxis—i. e., the order in which the leaves are arranged on the stem. This is of such a nature that each leaf stands some distance above and to one side of its next neighbor below, and if a line be drawn from the insertion of any one leaf through the points of insertion of those above, it will describe a spiral, and will eventually come to a leaf standing directly above the leaf started from. In doing this the spiral line will pass twice round the stem, and through the points of insertion of five leaves. This is shortly expressed by two fifths.
The previously homogeneous embryonic tissue in the section now shows certain patches of grayer, closer tissue, arranged round the center in a peculiar manner; these are transverse sections of the young vascular bundles—strands which at present are distinguished chiefly by the small diameter of their cells, whence the darker gray appearance.
These strands when young are called procambium strands. Their cells are distinguished from the other embryonic cells around by growing more in length and dividing less frequently across their length, and by growing less in breadth and dividing more often by longitudinal walls.
On transverse sections a little lower down there may be seen a number of elongated and curved patches of procambium, as shown in Fig. 9. On the section it will be noticed that the larger strands are so arranged that they inclose a five-angled mass of central tissue (the pith), the five comers pointing to the angles of the young stem to which the leaves are attached. At the corners or ends of the rays just referred to are in some cases two or three smaller strands. Now, the important point to apprehend first is that these strands at the corners (m, s) are the strands which pass directly into the leaves through the petioles, and it is necessary to be perfectly clear on this subject in order to understand much of what follows. For instance, the three strands marked m in Fig. 9, a (mm, ms, and ms in Fig. 10), pass directly into a given leaf, mm, in the middle, flanked by ms on either side; but this group is also accompanied on each side by another strand (marked s, s' in Fig. 9, a, and l, l in Fig. 10), so that five strands may be regarded as contributing to each corner of the section, the three middle ones running side by side up the midrib of the leaf and then branching out in a manner to be described subsequently.
It can be shown, moreover, that the larger curved strands, occupying the sides of the pentagon, are simply formed by the union of several of the smaller strands at different levels.
If, now, successively lower sections are cut of the very young shoot, and compared, or if the shoot is softened and dissected, it is possible to make out the course of these vascular bundle strands lower down; the course is somewhat complex, but the diagrammatic sketches in Fig. 11 will enable the reader to apprehend the chief points.
In the first place, the middle strand from a leaf, mm, passes vertically down in the angle of the young stem through five internodes (marked by the horizontal lines), turning to one side and becoming continuous in the fifth internode with a strand coming off from another leaf situated at another of the angles at a different level. The strands which stand next to this median one—one on each side (ms)—at first also pass
Fig. 10.—Diagram of the course of the bundles M, s, and s' of Fig. , as they pass out of the stem into the base of the leaf-stalk. mm is the median bundle, and ms, ms its two companions (M in Fig. 9, A); l, l are the lateral bundles s and s' of Fig. 9, A. The small branches fst go into the stipules. (After Frank.)
vertically down together with it, but at about the second or third internode below they break up into smaller strands, which again join with strands coming from other leaves situated at other nodes and angles.
If we again compare the figures, it will be seen that the three strands just traced come down in the angle of the stem, only turning aside lower down—the median strand mm, indeed, running actually in the angle through five internodes.
To right and left in Fig. 10 are seen two strands, marked l, l, and these run chiefly in what may be called the faces of the five-angled stem; only, at the node where the leaf we are considering is inserted, they turn in towards the leaf, and eventually they run into the sides of the petiole of the leaf as the so-called "lateral strands," or bundles.
Now, observation shows that these lateral strands (marked l, l², l³, etc., in the diagram,
Fig. 11.—Diagram of the course of the vascular bundles as they come down from the leaves into the stem. The horizontal dotted lines represent the levels of sucessive leaves; the triangular white area beneath the upper letter z is the insertion of a leaf. Each group of bundles form a leaf, as mm, ms, ms, etc. (see text), descends into the stem, and joins with the bundles from other leaves after running through several internodes. The other letters refer to the bundles from other leaves. (After Frank.)
It results from this that all the vascular bundles in the stem are simply composed of strands which run into the leaves on the one hand, and down the internodes on the other; and, as further comparison will show, all these bundles are continuous in the stem, since the lower ends of the strands are joined on to other strands.
Moreover, as an examination of the diagrams and figures shows, the main course of these bundles in the stem is approximately parallel—they run side by side down from the leaf insertion through two, three, or more internodes, and only bend aside to any great extent when they pass out into a leaf or to join with others. In the section (Fig. 9), for instance, all the little bundles at the angles and outside the ring are cut at levels where they have abandoned the larger bundles and are bending outward through the cortex to the leaves; lower down we should find them joining to the larger bundles at various levels, and running down with them, just as strands from leaves at higher levels are now conjoined to make up these larger bundles. The group of vascular bundles which passes into the stem from the insertion of a leaf is spoken of collectively as the "leaf-trace." Hence we see the leaf-trace of the oak consists of five bundles—one median, two lateral median, and two lateral; and since the phyllotaxis of the oak is two fifths, there will be twenty-five bundles in various stages of separation or conjunction coming down in the five internodes between any one leaf and the leaf vertically above it, as well as the parts of bundles from other leaves which are still continuing their course for a short time.
Now, since the main lengths of the course (in the stem) of these bundles is nearly vertically downward, with slight swerves to one side or another as the strands join, it is obvious that on the transverse section of the stem the bundles will appear arranged in a series round the center—in fact, they will form on the whole a more or less regular ring of bundles dividing off the pith from the cortical portions of the stem. Even in the very young condition (Fig. 9) we see bundles or groups of strands thus surrounding the pith, only the "ring" which they make is a sinuous one, so that the pith is five-rayed—a characteristic point in the oak. At a slightly later stage, as we shall see, this ring of bundles becomes more nearly circular from the gradual filling up of irregularities.
Before proceeding further it is necessary to make clear one or two other points. Since all the vascular bundles in the oak-stem are bundles which are common to the stem and leaf, they are termed "common bundles." We have seen that a given strand or bundle may run for part of its course simply side by side with another and separate from it; at other parts of the course the bundles may be united with others. In the case of the oak it will be clearly borne in mind that the individual or separate bundles of the leaf-trace pass into the stem at the node of insertion of the given leaf, and then run down side by side at a practically constant distance from the surface of the epidermis on the one hand, and the longitudinal axis of the pith on the other. At different levels below, at or very near nodes, these bundles turn aside laterally—i.e., in the tangential plane, and hence, still keeping their mean distance from the epidermis and pith, join with others.
This being understood, it is also obvious that on the whole the collection of vascular bundles in a young branch form a nearly cylindrical trellis-work or mesh-work symmetrically disposed between the pith and the cortex, and that the latter (cortex and pith) are in connection through the meshes between the interpectinating and concomitant vascular bundles. These radial connections of the pith and cortex are the primary medullary rays.
It will now be clear why we observe on transverse sections of the young stem taken across an internode the arrangement shown in Fig. 9. The vascular bundles are grouped in a ring round the pith, separating it off from the cortex and its covering the epidermis, and with those primary medullary rays which happen to have been cut running between the bundles.
If we now trace the vascular bundles of the leaf-trace in the other direction—that is, up into the leaf—their course is simple enough, as shown in Figs. 10 and 11. The five bundles run through the midrib and the stronger lateral ribs to the tips and edges of the leaf, first breaking up into several strands in the petiole and midrib, and then becoming finer and finer as they give off the lateral strands. The median bundle does little more than run directly through the leaf as the midrib, becoming finer and finer as it nears the apex. The two lateral median bundles behave in a somewhat curious way. We have already seen how large and flat they are at the leaf insertion (Fig. 10). Soon after entering the petiole they break up into several strands, two of which converge and take a course along the dorsal side of the midrib, thus nearly completing a cylinder of bundles inclosing a pith; moreover, the xylem portions of these bundles are all turned inward towards the pith.
The lateral bundles, coming obliquely into the leaf insertion, pass up the midrib side by side with the above, and, like them, break up into parallel strands. Before entering the midrib they give off small bundles (fst in Fig. 10) to the pair of minute stipules which flank the petiole. As the strands pass along the midribs and chief lateral ribs they interosculate in various degrees, and give off smaller side branches into the mesophyll of the leaf (see Chapter VI).
The veins which spring from the chief lateral ribs run towards one another and anastomose, giving off smaller veins which form a network in the area included by them. In the neighborhood of the leaf-margin, however, the smaller veins curve towards one another, and make arches convex towards the margin. In the finer meshes individual minute branches run to the center of a mesh and end there. Round the extreme edge of the leaf is a single vascular bundle; this receives small bundles from the above-mentioned arches, and also receives the ends of the midrib and the chief lateral ribs (cf. Fig. 1).
The vascular bundles of the axillary bud, which will eventually, of course, form a system like that already described on their own account, pass down and join the bundles of the parent axis as follows:
The bundles of each lateral half of the bud (Fig. 11, a a) pass down together between the bundles of the leaf-trace of the leaf from whose axil the bud arises, and the next lateral bundles of the stem with which the leaf-trace bundles are conjoined; the common strand formed by the bundles of each side of the bud then joins with a bundle coming down from another leaf. A few of the strands may also join to the bundles of the leaf-trace itself.
At the back or top side of the bud—i.e., the side next the stem which bears it—a few vascular bundles pass from the bud to the nearest strand (Fig. 11, z) this is the middle strand coming down from the leaf vertically above the bud—i.e., the sixth leaf up the stem. Knowing this, we of course know how the branch is joined to the stem. Several other small strands also are formed, as at z, to complete the filling up the gap, and these may be called completing bundles. These connecting and completing bundles enable the young shoot as it develops from the bud to inclose its own pith in a cylinder of vascular tissue continuous with that of the parent shoot.
We thus see that the vascular bundles form a connected system in the leaves, buds (i.e., young branches), and stem, and it only remains to add that they are joined below to those of the root-system, with which, in fact, they took origin in the very young embryo. Hence, if we were to remove the whole of the softer tissues of the oak-plant, we should have a model of it left in the form of a more or less open basket-work of vascular bundles. It is necessary to bear this in mind, as some important conclusions follow from it subsequently.