Читать книгу The Oak: A Popular Introduction to Forest-botany - Harry Marshall Ward - Страница 5

Table of Contents

the acorn and its germination—the seedling.

When the acorns are falling in showers from the oaks in October and November, everybody knows that each of the polished leather-brown, long, egg-shaped bodies tumbles out from a cup-like, scaly investment which surrounded its lower third at the broader end. Perhaps everybody would not be certain as to whether the detached acorn is a seed or a fruit, so I anticipate the difficulty by stating at the outset that the acorn is the fruit of the oak, and contains the seed within its brown shell; and I propose to commence our studies by examining an acorn, deferring the explanation of some minute details of structure until we come to trace the origin of the fruit and seed in the flower.

The average size of the fruit is about 15 to 20 mm., or nearly three quarters of an inch, long, by 8 to 10 mm., or nearly one third of an inch, broad at the middle of its length; the end inserted in the cup or cupule is broad and nearly flat, and marked by a large circular scar (Fig. 2, s) denoting the surface of attachment to the cupule. This scar is rough, and exhibits a number of small points which have resulted from the breaking ​of some extremely delicate groups of minute pipes, called vascular bundles, which placed the acorn in communication with the cup and the tree previous to the

Fig. 1.—Sprigs of oak, showing the habit and the arrangement of the acorns, etc., in September. (After Kotschy.)

​ripening of the former. At the more pointed free end of the acorn is a queer little knob, which is hard and dry, and represents the mummified remains of what was the stigma of the flower, and which lost its importance several months previously, after receiving the pollen.

The outer hard coat of the acorn is a tough, leather-brown polished skin, with fine longitudinal lines on it, and it forms the outer portion of the true covering of the fruit, called the pericarp (Fig. 2,p). On removing it we find a thin, papery membrane inside, adhering partly to the above coat and partly to the seed inside. This thin, shriveled, papery membrane is the inner part of the pericarp, and the details of structure to be found in these layers may be passed over for the present with the remark that they are no longer living structures, but exist simply as protective coverings for the seed inside.

The centre of the acorn is occupied more or less entirely by a hard brown body—the seed—which usually rattles about loosely on shaking the ripe fruit, but which was previously attached definitely at the broad end. A similar series of changes to those which brought about the separation of the acorn from the cup—namely, the shriveling up of the tiny connecting cords, etc.—also caused the separation of the seed from the pericarp, and we may regard the former as a distinct body.

Its shape is nearly the same as that of the acorn in which it loosely fits, and it is usually closely covered ​with a thin, brown, wrinkled, papery membrane, which is its own coat—the seed-coat, or testa (Fig. 2, t). The extent to which the testa remains adherent to the seed, or to the inner coat of the pericarp, and both together to the harder outer coat of the pericarp, need not be

Fig. 2.—Sections of acorns in three planes at right angles to one another. A, transverse; B, longitudinal in the plane of the cotyledons, (l); C, longitudinal across the plane of the cotyledons; c, cotyledons; t, testa; p, pericarp; s, scar, and r, radicle; pl, plumule. The radicle, plumule, and cotyledons together constitute the embryo. The embryonic tissue is at r and pl. The dots in A, and the delicate veins in B and C, are the vascular bundles.

commented upon further than to say that differences in this respect are found according to the completeness and ripeness of the acorn.

Enveloped in its testa and in the pericarp, then, we find the long acorn-shaped seed, which seems at first to be a mere horn-like mass without parts. This is not the case, however, as may easily be observed by cutting the mass across, or, better still, by first soaking it in water for some hours; it will then be found that the ​egg-shaped body consists chiefly of two longitudinal halves, separated by a median plane which runs through the acorn from top to bottom. These two halves, lying face to face so closely that it requires the above manipulation to enable us to detect the plane of separation (Fig. 2, l), are not completely independent, however; at a point near the narrower end each of them is attached to the side of a small peg-shaped body, with a conical pointed end turned towards the narrow end of the acorn. This tiny peg-shaped structure is so small that it may be overlooked unless some little care is exercised, but if the hard masses are completely torn apart it will be carried away with one of them.

The two large plano-convex structures are called the cotyledons or seed-leaves (Fig. 2, c), and they, together with the small peg-shaped body, constitute the embryo of the oak. The peg-shaped body presents two ends which project slightly between the two cotyledons beyond the points of attachment to them; the larger of these ends has the shape of a conical bullet, and is directed so that its tip lies in the point of the narrower part of the acorn; the other, and much smaller end, is turned towards the broader extremity of the acorn. The larger, bullet-shaped portion is termed the radicle (Fig. 2, r), and will become the primary root of the oak-plant; the smaller, opposite end is the embryo bud, and is termed the plumule (Fig. 2, pl), and it is destined to develop into the stem and leaves of the oak. If the observer takes the trouble to carefully separate the two ​large cotyledons, without tearing them away from the structures just described, he will find that each is attached by a minute stalk to a sort of ridge just beneath the tiny plumule; this ridge is sometimes termed the collar. He will also see that the plumule and radicle fit closely into a cavity formed by the two cotyledons, and so do not interfere with the very close fitting of their two flat faces.

Summing up these essential features of the structure of the ripe acorn and its contents, we find that the fruit contains within its pericarp (which is a more or less complex series of layers, of which the outermost is hard) the seed; that this seed comprises a membranous testa inclosing an embryo; and that the embryo is composed of two huge cotyledons, a minute radicle, and a still more minute plumule; and that the tip of the radicle is turned towards the pointed end of the acorn, lying just inside the membranes.

Leaving the details of structure of the membranes until a later period, when we trace their development from the flower, I must devote some paragraphs to a description of the minute anatomy and the contents of the embryo as found in the ripe acorn, so that the process of germination may be more intelligible. Thin sections of any portion of the embryo placed under the microscope show that it consists almost entirely of polygonal chambers or cells, with very thin membranous walls, and densely filled with certain granule-like contents. These polygonal cells have not their ​own independent walls, but the wall which divides any two of them belongs as much to one as to the other, and only here and there do we find a minute opening between three or more cells at the corners, and produced by the partial splitting of the thin wall. We may, if we like, regard the whole embryo as a single mass of material cut up into chambers by means of partition walls, which have a tendency to split a little here and there, much as one could split a piece of pasteboard by inserting a paper-knife between the layers composing it; what we must not do, is to suppose that these cells are so many separate chambers which have been brought into juxtaposition. In other words, the cell-wall separating any two of the chambers is in its origin a whole, common to both chambers, and the plane which may be supposed to divide the limits of each is imaginary only.

I have said that the embryo consists almost entirely of this mass of polygonal, thin-walled cells, and such is called fundamental tissue; but here and there, in very much smaller proportion, we shall find other structures. Surrounding the whole of the embryo, and following every dip and projection of its contours, will be found a single layer of cells of a flattened, tabular shape, and fitting close together so as to constitute a delicate membrane or skin over the whole embryo; this outer layer of the young plant is called the epidermis.

Whenever the cotyledons, or the radicle, or plumule are cut across transversely to their length, there are visible certain very minute specks, which are the cut ​surfaces of extremely delicate strands or cords of relatively very long and very narrow cells, the minute structure of which we will not now stay to investigate, but simply mention that these extremely fine cords, running in the main longitudinally through the embryo, are termed "vascular bundles" (Fig. 2, a). It may be shown that there is one set of them running up the central part of the radicle, starting from just beneath its tip, and that these pass into the two cotyledons, and there branch and run in long strands towards the ends of the latter.

The three sets of structures which have been referred to are called "tissues," and although they are still in a very young and undeveloped condition, we may say that the embryo consists essentially of a large amount of thin-walled cell-tissue of different ages, which is limited by an epidermal tissue and transversed by vascular tissue. At the tips of the radicle and plumule the cell-tissue is in a peculiar and young condition, and is known as embryonic tissue.

As regards the contents and functions of these tissues, the following remarks may suffice for the present. The polygonal cells of the fundamental tissue of the cotyledons are crowded with numerous brilliant starch grains, of an oval shape and pearly luster, and these lie imbedded in a sort of matrix consisting chiefly of proteids and tannin, together with small quantities of fatty substances.

In each cell there is a small quantity of protoplasm ​and a nucleus, but this latter is only to be detected with difficulty. Certain of the cells contain a dark-brown pigment, composed of substances of the nature of tannin; and small quantities of a peculiar kind of sugar, called quercite, are also found in the cells, together with a bitter substance.

In the main, the above are stored up in the thin-walled parenchyma cells as reserve materials, intended to supply the growing embryo or seedling with nutritious food; the starch grains are just so many packets of a food substance containing carbon, hydrogen, and oxygen in certain proportions, the proteids are similarly a supply of nitrogenous food, and minute but necessary quantities of certain mineral salts are mixed with these. The vascular bundles are practically pipes or conduits which will convey these materials from the cotyledons to the radicle and plumule as soon as germination begins, and I shall say no more of them here, beyond noting that each strand consists chiefly of a few very minute vessels and sieve-tubes. The young epidermis takes no part either in storing or in conducting the food substances; it is simply a covering tissue, and will go on extending as the seedling develops a larger and larger surface.

We are now in a position to inquire into what takes place when the acorn is put into the soil and allowed to germinate. In nature it usually lies buried among the decaying leaves on the ground during the winter, and it may even remain for nearly a year without any ​conspicuous change; and in any case it requires a period of rest before the presence of the oxygen of the air and the moisture of the soil are effective in making it germinate—a fact which suggests that some profound molecular or chemical changes have to be completed in the living substance of the cells before further activity is possible. We have other reasons for believing that this is so, and that, until certain ferments have been prepared in the cells, their protoplasm is unable to make use of the food materials, and consequently unable to initiate the changes necessary for growth.

Sooner or later, however, and usually as the temperature rises in spring, the embryo in the acorn absorbs water and oxygen, and swells, and the little radicle elongates and drives its tip through the ruptured investments at the thin end of the acorn, and at once turns downward, and plunges slowly into the soil (Fig. 3). This peculiarity of turning downward is so marked that it manifests itself no matter in what position the acorn lies, and it is obviously of advantage to the plant that the radicle should thus emerge first, and turn away from the light, and grow as quickly as possible towards the center of the earth, because it thus establishes a first hold on the soil, in readiness to absorb water and dissolve mineral substances by the time the leaves open and require them.

The two cotyledons remain inclosed in the coats of the acorn, and are not lifted up into the air; the developing root obtains its food materials from the stores ​in the cells of the cotyledons, as do all the parts of the young seedling at this period. In fact, these stores in

Fig. 3.—I. Longitudinal section through the posterior half of the embryo, in a plane at right angles to the plane of separation between the cotyledons (slightly magnified). II. Germinating embryo, with one cotyledon removed. III. Acorn in an advanced stage of germination. a, the scar: s, pericarp; sh, testa; b, plumule; st, petioles of cotyledons, from between which the plumule, b, emerges; hc. hypocotyl; c, cotyledons; f, vascular bundles; w, radicle (primary root) ; w', secondary roots. Root-hairs are seen covering the latter and the anterior part of the primary root in III. (After Sachs.)

​the cotyledons contribute to the support of the baby plant for many months, and even two years may elapse before they are entirely exhausted.

When the elongated radicle, or primary root, has attained a length of two or three inches in the soil, and its tip is steadily plunging with a very slight rocking movement deeper and deeper into the earth, the little plumule emerges from between the very short stalks of the cotyledons (Fig. 3, st), which elongate and separate to allow of its exit, and grows erect into the light and air above ground. It will be understood that this plumule also is living at the expense of the food stores in the cotyledons, the dissolved substances passing up into it through the tiny vascular bundles and cells, as they have all along been passing down to the growing root through the similar channels in its tissues.

The plumule—or, as we must now call it, primary shoot—differs from the root not only in its more tardy growth at first, but also in its habit of growing away from the center of gravitation of the earth and into the light and air; and here, again, we have obviously adaptations which are of advantage to the plant, which would soon be top-heavy, moreover, if the shoot were far developed before the root had established a hold-fast in the soil.

The little oak shoot is for some time apparently devoid of leaves (Fig. 4), but a careful examination shows that as it elongates it bears a few small scattered scales, like tiny membranes, each of which has a very ​ minute bud in its axil. When the primary shoot has attained a length of about three inches there are usually two of these small scale-leaves placed nearly opposite one another close to the tip, and a little longer and narrower Fig. 4—Germinating acorn, showing the manner of emergence of the primary shoot, and the first scales (stipules) on the latter. (After Rossmässler) than those lower down on the shoot; from between these two linear structures the first true green foliage leaf of the oak arises, its short stalk being flanked by them. This first leaf is small, but the tip of the shoot goes on elongating and throwing out others and larger ones, until by the end of the summer there are about four to six leaves formed, each with its minute stalk flanked by a pair of tiny linear scales ("stipules," as they are called) like those referred to above.

Each of the green leaves arises from a point on the young stem which is a little higher, and more to one side, than that from which the lowermost one springs; hence a line joining the points of insertion of the successive leaves describes an open spiral round the shoot axis—i. e., the stem—and this of such a kind that when the spiral comes to the sixth leaf ​upward it is vertically above the first or oldest leaf from which we started, and has passed twice round the stem.

At the end of this first year, which we may term the period of germination, the young oak-plant or seedling has a primary root some twelve to eighteen inches long, and with numerous shorter, spreading side rootlets, and a shoot from six to eight inches high, bearing five or six leaves as described, and terminating in a small ovoid bud (Figs. 3 and 4). The whole shoot is clothed with numerous very fine soft hairs, and there are also numerous fine root-hairs on the roots, and clinging to the particles of soil. The tip of each root is protected by a thin colorless cap—the root-cap—the description of which we defer for the present.

About May, in the second year, each of the young roots is elongating in the soil and putting forth new root-hairs and rootlets, while the older roots are thickening and becoming harder and covered with cork; and each of the buds in the axils of the last year's leaves begins to shoot out into a branch, bearing new leaves in its turn, while the bud at the end of the shoot elongates and lengthens the primary stem, the older parts of which are also becoming thicker and clothed with cork. And so the seedling develops into an oak-plant, each year becoming larger and more complex, until it reaches the stage of the sapling, and eventually becomes a tree.