Cotyledon

“ The shape of the seed • seems to have an influence on the shape of J| the cotyledons. Where the cotyledons are narrow and lie straight in a long, narrow seed, the relation is simple, but such cases are few. Often narrow cotyledons are found coiled in orbicular seeds. In many broad seeds we find two fleshy cotyledons laid face to face, and occupying almost the whole of the seed. In the nearly spherical radish seed the cotyledons are laid face to face and then folded along the middle. In other species one cotyledon is larger than the other, or the halves of each cotyledon are unequal ”

“ In many Monocotyledons the terminal cell forms the cotyledonary portion alone of the shoot of the embryo, its axial part and the root being derived from the adjacent cell; the cotyledon is thus a terminal structure and the apex of the primary stem a lateral one—a condition in marked contrast with that of the Dicotyledons. In some Monocotyledons, [v.02 p.0012] however, the cotyledon is not really terminal. The primary root of the embryo in all Angiosperms points towards the micropyle. ”

“ Out of this the thick stem has issued; and we find that it is only the cotyledon of the seed, that has prolonged its base in the process of germination, in order to throw up, clear of the nut, the plumule and radicle.
We look at the great nut, and find, on the woody exterior of the fibrous pericarp, at the side [Pg 297] opposite to that whence issues the cotyledon, a broad scar. What is this? It is the mark left by the severance of a footstalk, which united the fruit to the parent plant. ”

“ In Maize or Indian Corn (Fig. 66-70) , the embryo is more developed in the seed, and its parts can be made out. It lies against the starchy albumen, but is not enclosed therein. The larger part of it is the cotyledon, thickish, its edges involute, and its back in contact with the albumen; partly enclosed by it is the well-developed plumule or bud which is to grow. For the cotyledon remains in the seed to fulfil its office of imbibing nourishment from the softened albumen, which it conveys to the growing sprout ”

“ The stem and the cotyledon.—Notice that a rod, somewhat thicker than a root, grows out near the origin of the roots, and curves upwards towards the light. When this is about an inch long on a maize seedling, slit it open carefully, and observe that it consists of a pale outer sheath and a green core. The sheath is the single cotyledon; the green core is the young stem enclosed in a young foliage leaf. Cut open the grain and notice how the endosperm has shrivelled. As the seedlings become larger watch the young stem growing out at the end of its sheathing cotyledon. ”

“ Leaves and cotyledons assume their nocturnal position by two means, by the aid of pulvini and without such aid. In the former case the movement continues as long as the leaf or cotyledon remains in full health; whilst in the latter case it continues only whilst the part is growing. Cotyledons appear to sleep in a larger proportional number of species than do leaves. In some species, the leaves sleep and not the cotyledons; in others, the cotyledons and not the leaves; or both may sleep, and yet assume widely different positions at night. ”

“ This begins as soon as the cotyledon has reached the light, and thus the formation of fibres of woody matter takes place, which fibres descend from the cotyledon to the radicle. Meanwhile the formation of other cells of the plumule takes place, until the first leaf is formed, when other fibres of wood are sent down, and so on for every leaf, so that the number of woody [Pg 123] fibres which form the trunk of a tree is in proportion to the number of leaves which that tree has borne, from which we come to the conclusion that the size of the trunk of a tree is the sum of all its branches. ”

“ The cotyledon of Primula sinensis moved in closely analogous manner, as do those of a Cassia. Hence I expect to find such movements very general with cotyledons, and I am inclined to look at them as the foundation for all the other adaptive movements of leaves. They certainly are of the so-called sleep of plants. ”

“ The Embryo of seeds that have a single cotyledon, or none at all, is peculiarly simple, without any notch or lobe, and is named by Gærtner Embryo monocotyledoneus.
Cotyledones, the Cotyledons or Seed-lobes, are immediately attached to the Embryo, of which they form, properly speaking, a part. They are commonly two in number ”

“ It should be noticed that there is only one cotyledon, and here the point to emphasise is, that the bean and sunflower live on the food contained in, or made by, the cotyledons; the maize on the food present in the endosperm.
The seedlings three weeks old should then be compared with those already observed, the differences in length of radicle and plumule being noted.
The observation of these seedlings will naturally suggest the subject of growth. What is growth? ”

“ Fig. 51. Zea mays: circumnutation of cotyledon, traced on horizontal glass, from 8.30 A.M. Feb. 4th to 8 A.M. 6th. Movement of bead magnified on an average about 25 times.
Zea mays (Gramineæ) .—A glass filament was fixed obliquely to the summit of a cotyledon, rising .2 of an inch above the ground; but by the third morning it had grown to exactly thrice this height, so that the distance of the bead from the mark below was greatly increased, consequently the tracing (Fig. 51) was much more magnified on the first than on the second day. ”

“ Correspondingly, their caulicle does not lengthen to elevate them above the surface of the soil; the growth below the cotyledons is nearly all of root. It is the little plumule or bud between them which makes the upward growth, and which, being well fed by the cotyledons, rapidly develops the next pair of leaves and raises them upon a long internode, and so on. The cotyledons all the while remain below, in the husk of the fruit and seed, and perish when they have yielded up the store of food which they contained. ”

“ In another natural genus, Oxalis, the cotyledons of O. Valdiviana, rosea, floribunda, articulata, and sensitiva are pulvinated, and all move at night into an upward or downward vertical position. In these several species the pulvinus is seated close to the blade of the cotyledon, as is the usual rule with most plants. Oxalis corniculata (var. Atro-purpurea) differs in several respects; the cotyledons rise at night to a very variable amount, rarely more than 45° ”

“ Application of light indicated by arrows; light acting from below retards, acting from above accelerates the movement. The last part of the curve in each shows recovery on the stoppage of light.
Figure 189 exhibits the effect of light applied alternately above or below the cotyledon of Cucurbita Pepo. On account of the more rapid growth of the lower side, the cotyledon was exhibiting a hyponastic up-movement. Application of light from above enhanced the existing rate [Pg 530] of movement, whereas light acting from below retarded the movement. ”

“ We may here mention another point of similarity between sleeping leaves and cotyledons, namely, that some of the latter (for instance, those of Cassia and Githago) are easily affected by the absence of light; and they then either close, or if closed do not open; whereas others (as with the cotyledons of Oxalis) are very little affected by light. In the next chapter it will be shown that the nyctitropic movements both of cotyledons and leaves consist of a modified form of circumnutation. ”

“ When the seed is broken the cotyledons are found adherent to the testa, with a large cavity between them. The air thus included causes the seeds to float on water, but they sink immediately when broken. After digestion for some hours in warm water, the testa having been previously cracked, the whole seed softens and swells so that its structure may be easily studied. Each cotyledon is then seen to be marked on the hilum-side by a long shallow furrow, at one end of which, just below the micropyle, lies the plumule and radicle. ”

“ If the embryo be held with the edge of the cotyledons towards the eye, it is the left blade which comes from the back of the radicle, and thus has the longer turn to make. Cakile Americana, Nutt., resembles Lepidium Virginicum in these particulars, except that the “radicular” portion of the cotyledons is relatively much shorter, and in one instance I found the blades of the cotyledons almost spirally bent over the radicle, so as to pass, as it were, through an incumbent stage. ”

“ Fruit small obliquely ovate pointed drupes, each raised on a slender stalk which appears after flowering; the spadix itself also then raised on an elongated thread-form peduncle. Embryo ovoid, with a short and pointed plumule from the upper end, by the side of the short cotyledon.—Marine herbs, growing under water, with long and thread-like forking stems, and slender almost capillary alternate leaves, sheathing at the base. Flowers rising to the surface at the time of expansion. ”

“ In other instances single cylindrical leaves arose from the earth, where no bulbs were to be found; these cylindrical leaves were thought to arise from seeds, which, if it was a fact, would prove that the plant vegetated with but one cotyledon. In a short time the vessel was crowded with the seeds of the limosella raised by the cotyledons. ”

“ Flowering plants bear a seed containing an embryo, with usually one or two cotyledons, or seed-leaves; while in flowerless plants there is no seed and therefore no true cotyledon. The term is synonymous with Cryptogams, by which it was replaced in later systems of classification. ”

“ Darwin accounts for this from the damage which the cotyledons may receive from keeping, by which their power of nourishing the infant plant, at its first germination, is lessened, and it becomes stunted and dwarfish through its whole duration.
Dr. Thomson of Edinburgh, in his System of Chemistry, , has published a very satisfactory explanation of one part of the functions of the cotyledons. Several philosophers have discovered that very soon after the seed begins to imbibe moisture, it gives out a quantity of carbonic acid gas, even though no oxygen gas be present. ”

“ On picking the embryo carefully out of the seed coats, however, I find a conformation that in a measure removes the difficulty. In the speciess which I have examined, viz., the present, L. ruderale, L. campestre, and, perhaps, L. intermedium, the cotyledons are continued, in the form of petioles, about half way down the radicle; the cotyledons, in fact, being transversely folded upon themselves, as stated and illustrated in Gray's Genera in the case of Subularia and Senebiera, the genera immediately preceeding Lepidium. ”

“ We will therefore speak of a cotyledon and hereafter of a leaf as sleeping, only when it rises at night to an angle of about 60°, or to a still higher angle, above the horizon, or sinks beneath it to the same amount. Not but that a lesser diminution of radiation may be advantageous to a plant, as in the case of Datura stramonium, the cotyledons of which rose from 31° at noon to 55° at night above the horizon. The Swedish turnip may profit by the area of its leaves being reduced at night by about 30 per cent., as estimated by Mr. ”

“ The embryo connects the “cotyledons” or halves of the seed, and this develops into a tiny rootlet or “radicle,” and upwards into the stem, the commencement of which is known in botany as a “plumule.” The rootlet seeks nourishment from the ground. The albumen secreted in the cotyledons feeds the embryo, until (in some cases) it is exhausted and they die away. In other cases they grow up and obtain food for the young plant in the air. Some plants have (like wheat) only one seed-leaf, or cotyledon; and these kinds are called monocotyledons, or endogens, in which the growth is upright. ”

“ When studying the germination of the American and Asiatic Rhizophoras in Fiji, I observed that the neck of the cotyledonary body did not begin to form, nor the inclosed plumular bud to show signs of differentiation, until the hypocotyl had protruded about 41⁄2 inches with R. mangle, and between 6 and 7 inches with R. mucronata. The neck of the cotyledonary body then proceeds to grow in length, pushing before it the plumular end of the embryo-seedling, which it surrounds as a sheath. ”

“ If so, the lupine and vegetable-marrow seeds evidently rely partly upon their stored food and partly upon setting the cotyledons to work as leaves, whilst the plant is still very young.
The true leaves.—The cotyledons are really makeshift leaves, which are already formed in the seeds. Even when they expand and become green they do not live long, but as soon as the next few leaves are well established, shrivel up and wither. The true or “foliage” leaves first make their appearance as a bud which surrounds the growing point of the stem. ”

“ They comprised the half inch, or inch united portions before referred to. If these radicular portions of the seed were of the nature of root rather than of stem, we might expect to see lateral fibres push from them, as we do see from the true roots, which start out below the union. But these parts are as free from rootlets as any portion of the true stems above the cotyledon points, indicating, as had been suggested, that their properties were rather of stem than of root. ”

“ The nutriment within the cotyledons is then quickly transferred to the hypocotyl or radicle, which thus becomes developed into a fusiform tuber. The fact of tubers being formed by the foregoing three widely distinct plants, makes us believe that their protection from animals at an early age and whilst tender, is one at least of the advantages gained by the remarkable elongation of the petioles of the cotyledons, together with their power of penetrating the ground like roots under the guidance of geotropism. ”