The Oak: A Popular Introduction to Forest-botany. Harry Marshall Ward
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Название: The Oak: A Popular Introduction to Forest-botany
Автор: Harry Marshall Ward
Издательство: Bookwire
Жанр: Математика
isbn: 4064066442682
isbn:
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 СКАЧАТЬ