The Oak (Ward)/Chapter VI
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The Seedling and Young Plant (continued). Buds and Leaves
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the seedling and young plant (continued).
The Buds and Leaves.
The buds of the oak—those in the leaf-axils as well as those at the tips of the young shoots—are characteristically short and broad ovoid bodies, consisting of numerous overlapping brown scales covered with short, silky hairs, especially at the margins (Fig. 19). These scales are really the stipules of arrested leaves, as is shown by the proper leaf-blades being developed as well under certain circumstances, such as when nutritive materials are directed to the young buds. The same morphological fact is also shown by the position of the inflorescences and young leaves higher up in the bud, for they spring from between the scales, and not from their axils proper (see Fig. 32). It is of the highest importance to understand that a bud is simply the young state of a shoot, and that it consists of the growing-point of the shoot enveloped by closely-folded leaf structures. In the oak the buds are already formed before the end of June, and on looking closely into the axils of the leaves on the young shoots—which have by that time ceased to elongate to any considerable extent farther—they may be seen as small, green, hairy bodies. During the remainder of the summer the chief changes going on in these buds is a slow swelling, due to the
Fig. 19.—A. End of a branch of oak showing the characteristic winter buds. B. A group of buds (slightly magnified): a, bud-scales; d, leaf-scars. C. The same, in longitudinal section: a, bud-scales (stipules); b, young leaves; c, vascular bundles; d, leaf-scars. (Prantl and Hartig.)
gradual storing up of nutritive materials in the pith and growing-point and to the slow division of the cells.
A vertical section through the bud at the end of the autumn shows the following structures (Fig. 19, C): A conical growing-point, consisting of embryonic tissue, occupies the center; around this, arranged in a close spiral, are several young rudiments of foliage leaves, each consisting of meristem, the cells of which are undergoing divisions. The youngest leaf is next the apex of the cone—i.e., the order of development is acropetal—and each is folded with the upper surfaces of each half in contact; two extremely minute stipules accompany each leaf. Lower down on the cone come the numerous (about thirty) overlapping scales, and between several pairs of the upper of these the male inflorescences develop. The female inflorescences are developed in the axils of two or three of the above-described true leaves in a terminal bud; they are not normally formed in the lateral buds of the shoot (see Chapter IX).
All the leaves of the shoot may have such buds formed in their axils during the summer, but only some of them develop in the following spring; it is the buds in the axils of the lower leaves of the shoot which usually come to nothing.
The normal course of events is that the bud-scales (stipules) become dry, and the protected growing-point, with its rudimentary leaves and flowers, passes into a dormant condition lasting through the winter; but it is a very common event, especially in a wet autumn following a dry, hot summer, to find the winter buds beginning to shoot out in August, and not passing into the prolonged state of dormancy. Such shoots are known as Lammas shoots. In some districts the oak forms numbers of these Lammas shoots every year, and the tendency to produce them seems to be capable of being inherited.
The process of sprouting, or putting forth the shoot from the bud, is the same in all the cases. As the temperature and other conditions improve in the spring, for instance, the process of cell-division in the growing-point (and its derivatives, the young leaves, etc.) goes on rapidly, and the stores of nourishment already there and in the pith and other tissues close at hand are used up. This originates a series of currents of food materials setting slowly towards these centers of consumption from other parts of the tree, and very soon the numerous cells developed begin to absorb water with relatively enormous rapidity and vigor. This brings about two chief changes—the rapid elongation of the parts of the cone situated between the points of insertion of successive leaves (i.e., the internodes), and the almost simultaneous expansion of the hitherto small and folded leaves. Thus the rapid extension of the shoot is due almost entirely to the energetic absorption of water into cells for the most part already in existence. The chief changes which follow consist in the perfection of the structures—the development and thickening of vascular tissues, cell-walls, etc.
This process of rapid extension does not occur in the internodes between the bud-scales, or, at any rate, to a slight degree only, just sufficient to enable the shoot to throw the scales off; hence the base of the outgrown shoot shows a number of small scars in a close spiral. These scars of the stipular bud-scales, like those of fallen leaves, exhibit the points of rupture of the vascular bundles which ran across from the bundles of the bud-axis. It only remains to point out that the buds vary in size and vigor according to the age and condition of the tree; the buds on oaks less than fifty years old very rarely have inflorescences developed in them, and I shall defer the consideration of these till we come to the flower.
The mature leaf of the oak (Fig. 20) is obovate in general outline, with rather deep sinuses cutting the margin on each side into about six or eight rounded lobes; the apex is rounded or blunt, and some variation occurs in the degree of incision between the lobes. The base either tapers slightly into an evident petiole, or it is prolonged on either side of a very short petiole so as to form small auricles. In the commonest variety the margins and surfaces of the leaf are quite smooth, but the raceform known as Quercus sessiliflora has the young leaves pubescent beneath.
The venation consists of a midrib running from base to apex, and pinnate lateral ribs running from the mid-rib at an angle of about forty-five degrees to the tip of each lobe, the points of origin being alternate or nearly opposite, and the angle referred to subtending forward. These principal ribs are prominent below, but not at all so above. The leaf-tissue (mesophyll) between these is permeated by numerous smaller vascular bundles united into an irregular network, but so arranged that they leave between them nearly equal small areas not traversed bv bundles.
Fig. 20.—Sprigs of oak, showing the habit and the arrangement of the acorns, etc., in September. (After Kotschy.)
When young, the leaves are red, gradually becoming a bright apple-green, and finally—in the autumn—becoming russet-brown in color. Young oaks retain their dead leaves till far into the winter, and even old trees usually have some leaves attached till January. The young leaves secrete small quantities of sweet liquid on the superior face of the lamina, and are much visited by bees and wasps; this honey must come through the membrane. As the leaves approach maturity the lamina becomes bright and hard.
The arrangement of the leaves is expressed by the fraction two fifths, as already described, each node giving off one leaf at an open angle, the points of insertion being so arranged that a line drawn from the insertion of a given lower leaf, and joining it to the points of insertion of those above, passes twice round the twig before we arrive at the leaf situated vertically above the one started from, and this upper leaf is the sixth above. Although this is the commonest and normal arrangement, however, other dispositions are occasionally met with on the same plant. The young leaves are folded in the bud in such a manner that the two halves of the lamina lie one on the other, the upper surfaces being in contact (conduplicate vernation), the margins being therefore turned upward.
In order to understand the structure of the leaf, let us look at a section cut neatly across the midrib and lamina, and examined with the microscope. It is found to consist of three principal parts—an epidermis above and below, and all round the margins, and therefore over the whole of the leaf; this epidermis is, in fact, a continuation of that of the young shoot-axis, and envelops the whole of the remaining leaf-tissues. Inside this we have the main mass of the leaf substance—called the mesophyll—consisting of thin-walled cells arranged in a peculiar manner, and containing (in addition to less obvious structures) large numbers of green chlorophyll corpuscles; it is the predominance of these corpuscles which causes the leaves to appear uniformly green. Here and there we see vascular bundles, imbedded, as it were, in the mesophyll, cut across in various directions; and when it is remembered that these vascular bundles constitute the venation of the leaf, this phenomenon is easily explained.
As we have already seen, the vascular bundles of the venation (Fig. 20) are simply the much-branched and thinned-off upper ends of the vascular bundles from the shoot-axis, the lower ends of which join the vascular system of the latter lower down. Now the next point to be clearly apprehended is that these vascular bundles of the leaves have the double duty of supporting the flattened
Fig. 22.—Sections across the leaf of oak. A. Slightly magnified and semi-diagrammatic, to show the general arrangement of the principal vascular bundles as seen cut across: m, midrib: e, marginal veins; s, lateral branches of midrib. Other smaller veins scattered between. B. A highly magnified vertical section of part of the above at a place free from vascular bundles: u, upper epidermis, with cuticle, c; p, palisade cells; ch, chlorophyll corpuscles, only drawn in a few cells; m, spongy tissue of mesophyll; i.s, intercellular passages communicating with the stoma, st, in the lower epidermis, l.
mass of leaf-tissue, and of carrying to and from its cells the water from the roots and the organic substances formed in the cells of the leaves. The water, with salts in solution, coming from the soil after it has been absorbed by the root-hairs, passes up the wood (xylem) of the roots and stem, through the vessels of the petioles and leaf-venation, and is finally distributed to the cells of the mesophyll; the substances formed in these cells then pass down by the phloëm (sieve-tubes, etc.) of the venation and leaf-stalk, and thence are distributed to other parts of the plant.
Now let us look at the mesophyll which these vascular bundles support and serve as conduits for. It consists of two distinct parts (Fig. 22). Beneath the upper epidermis, the cells of which are fitted closely together without intercellular spaces and are devoid of chlorophyll corpuscles, there are one or two rows of vertical sausage-shaped cells, closely arranged like the wooden railings of a complete palisade—consequently they are termed the palisade cells. The lower moiety of the mesophyll, on the other hand, is composed of irregular cells with large intercellular spaces between them, and this loose, spongy tissue, as it is aptly called, abuts below on the lower epidermis. Both the palisade cells and those of the spongy tissue contain numerous chlorophyll corpuscles, as said.
This lower epidermis is worth a few minutes' consideration. It, like the upper epidermis, is also composed chiefly of closely fitting cells devoid of chlorophyll corpuscles, excepting that here and there we notice pairs of smaller cells containing chlorophyll—each pair with a minute gap between them, and the gap communicates with the intercellular air-cavities between the cells of the spongy mesophyll (Fig. 22, st). If we remove a piece of this epidermis, and look at it as laid flat (instead of in section) under the microscope, we find that these pairs of small cells are shaped somewhat like a small mouth, the two curved lips of which are formed by the two cells just mentioned, and the orifice of which is the gap just referred to (Fig. 23). These two lips are called the guard-cells, and the whole apparatus is termed a stoma. It is necessary to realize two great facts about these stomata on the under surface of the leaf: firstly, there are several hundreds of thousands of them on an oak-leaf, each square millimetre having from 300 to 350 of them scattered over it; and, secondly, each one can open or close its little aperture by the approximation or divarication of the inner concave sides of the curved guard-cells.
If this is clear, it will be readily understood that these stomata can regulate the amount of water passing off by evaporation from the walls of the millions of cells of the mesophyll, especially if the further fact is borne in mind that water-vapor scarcely passes at all through the close-fitting epidermis cells themselves.
We are now in a position to form a sort of picture of the mechanism of the shoot and root in regard to this matter. The root-hairs absorb water from the soil, and in this water there are dissolved small quantities of the soluble salts of the earth—chiefly sulphates, nitrates, and phosphates of lime, magnesia, and potash—just as there are in ordinary well-water. This extremely dilute solution passes into the root-fibers and up through the
Fig. 23.—A. A small piece of the lower epidermis removed (and highly magnified) to show the stomata, g; h, minute hairs. The guard-cells contain chlorophyll corpuscles, whereas the ordinary epidermal cells do not. B. A stoma in vertical median section, cut across its longer axis: a, intercellular space: g, guard-cell with chlorophyll corpuscles; s, orifice of stoma.
vessels, etc., of the vascular bundles of the roots, collecting into the larger and larger channels until it reaches the stem; here it passes up the xylem to the branches, petioles, and leaf-venation—always in the wood—and is finally distributed to the mesophyll cells, which absorb it and evaporate the greater part of the water into the intercellular passages communicating with the outer air through the stomata.
Two points need notice here. The first is that this absorption and evaporation in the mesophyll constitute a cause of the upward movement of the water in the vascular bundles—a movement which is propagated through the whole stem until it makes itself effective even in the roots. The exact mechanism of the movement in the stem itself is too complex for discussion here; but I may sum up the matter by saying that the disappearance of the water at the surfaces of the leaves starts a series of flows in directions of least resistance towards the mesophyll, and as long as the evaporation goes on more water flows into the cells, to replace that lost, from the vessels of the stem, when the water-columns are supported and moved partly by capillarity and by the air-bubbles in the cavities, and partly by a peculiar co-operation of the living cells of the medullary rays. The second point referred to above is that the evaporation from the mesophyll cells will be the more rapid in proportion as the air outside is drier and the stomata wide open; and the more energetic this evaporation is, the more salts the mesophyll cells will acquire in a given time, because, of course, the salts do not pass away in the evaporated water but are left in the cells. It has been calculated that an oak-tree may have 700,000 leaves, and that 111,225 kilogrammes of water may pass off from its surface in the five months from June to October, and that 226 times its own weight of water may pass through it in a year.
Now comes the question, What are the salts needed for that so much mechanism should be expended on their accumulation? To answer this, we must look at the mesophyll cells a little more closely. Each of these consists of a thin cellulose cell-wall, lined with colorless protoplasm, which incloses a large sap-cavity (vacuole); in the protoplasm are imbedded a number of bright-green, rounded chlorophyll corpuscles, a relatively large nucleus, and a few less conspicuous granules, etc. The cell-sap contains various substances dissolved in water. Some of these substances are salts and other materials ready to be made use of; others are, so to speak, waste products or worked-up materials that are going to be got rid of, or sent to places where they will be made use of, respectively.
In the colorless protoplasm which lines the interior of the cell-wall and surrounds the cell-sap we find a nucleus and the chlorophyll corpuscles, as said, and a few words must be devoted to the latter. Each chlorophyll corpuscle consists of a rounded mass of proto-plasmic substance of somewhat spongy texture, containing the peculiar green body, chlorophyll, imbedded in it as in a matrix. These chlorophyll corpuscles are living organs, and they require food materials—water, oxygen, etc.—for the support of their life processes, just as do the other living parts of the cell—e.g., the colorless protoplasm and nucleus. They obtain these from the cell-sap, through the agency of the colorless protoplasm in which they reside.
In order that they may perform their functions properly, however, it is essential that they be exposed to light; this is effected by their being in cells which are disposed in thin layers, such as we have seen the mesophyll of the leaf to be. In fact, the flat, thin, expanded form of the leaf is a direct adaptation to the end that these chlorophyll corpuscles shall be properly illuminated by the sunlight; moreover, the large intercellular passages which communicate by thousands of stomata with the atmosphere insure their being thoroughly aerated. In addition to allowing the free access of the oxygen of the air, moreover, these intercellular passages admit of the small quantities of carbon dioxide in the atmosphere also reaching the chlorophyll corpuscles. Oxygen and carbon dioxide, therefore, are found dissolved with the other materials in the cell-sap which saturates the protoplasm and reaches the chlorophyll corpuscles.
These facts premised, we are in a position to follow generally the astounding transformations which go on in these millions of chlorophyll corpuscles in the oak-leaf. Carbon dioxide and water exist side by side in the protoplasm of the chlorophyll corpuscle, and rays of sunlight—i.e., energetic vibrations of the ether which pervades the universe—penetrate into the system. By means of the energy thus derived from the sun, the molecules of carbon dioxide and water are broken up in the meshes of this chlorophyll corpuscle, and experiments prove that the chlorophyll substance plays the part of the "trap to catch a sunbeam." We are not concerned with the hypothetical explanations offered for all the details of this remarkable process, but the present position of science enables us to say that, be these what they may, the chlorophyll corpuscle gains energy from the sun, and brings this energy to bear on the carbon dioxide and water in such a way that it does work in tearing asunder their molecules in the substance of the corpuscle. Then a curious series of results follow. The carbon, oxygen, and hydrogen undergo new re-arrangements, which amount finally to this—the substance known as starch, and consisting of carbon, hydrogen, and oxygen, is built up in the form of granules in the chlorophyll corpuscle, and the surplus oxygen escapes into the sap and finds its way to the intercellular passages, and thence through the stomata into the atmosphere.
It will be obvious from the foregoing that the granules of starch represent so much matter (especially carbon) obtained from the atmosphere outside the plant, and so much energy obtained from the sun; each granule may therefore be regarded as a packet of stored energy and matter won from the external universe.
The limits of this little book will not allow of my going into details concerning the use which the plant makes of this starch, and it must suffice to say that the starch serves as the basis of all the constructive materials used by the tree. Thus it is converted into a soluble form, and combined with nitrogen, phosphorus, sulphur, etc. (obtained from the earth-salts), to make new protoplasmic materials, and it passes down from the leaves to nourish all the living cells that require it, in the embryonic tissue at the apex of the roots, and that at the apex of the stem and branches, buds, etc., and some of it passes to nourish the cambium cells, the developing flowers, acorns, etc.; in short, wherever new organic material is needed it is supplied from these stores formed by the green leaves waving in the sunshine. If we reflect that the little embryo in the acorn starts its life with only a minute store of starch and proteids in its cotyledon, and that all the tons of organic material (chiefly wood) found in an old oak-tree have been super-added to this by the action of the leaves—the small proportion of salts taken up by the roots being quite inconsiderable in comparison—we obtain some idea of the enormous gain of matter and energy from the outside universe which goes on each summer.