The Oak: A Popular Introduction to Forest-botany. Harry Marshall Ward

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СКАЧАТЬ 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 СКАЧАТЬ