In the Monocotyledonous division, which may be represented by a grain of wheat ([Pl. I.] fig. 53), the single cotyledon forms a minute sheath (a), enclosing the plumule (b), the radicle (c) being here but little developed at first, the greater part of the grain consisting of the albumen (d); the grain should be softened in water before examination. In the germinated grain the cotyledon appears as a pale sheath, surrounding the convolute green leaves of the plumule; which may be best seen in a transverse section ([Pl. I.] fig. 48).
Fertilization.—A few words must now be said regarding the formation of seeds, and the action of the pollen-tubes in the process of fertilization.
In the earliest stages of growth, the young seeds, or ovules as they are called, appear as little buds, arising from the inner wall of the ovary; and the part from which they arise is called the placen´ta (placenta, a cake). In chickweed ([Pl. I.] fig. 41 c), the placenta forms a central column; and when the ovules are a little older, they are found to have separated somewhat from the placenta, but retaining a connexion by means of a little cord or stalk, termed the funic´ulus (funis, a cord). The ovules may be readily found in the ovary or young pod of a wall-flower, the placentas forming four lines, running longitudinally down the interior of the pod.
In this early condition the ovule consists of a mass of cellular tissue; and as new formations are soon added to it, it is termed in this state the núcleus. Around the nucleus are then formed two coats, an outer, called the prímine, and an inner, termed the secun´dine. These coats or membranes are open at one end, so as to leave a passage down to the apex of the nucleus; the opening is called the forámen. These structures are well seen in the ovule of the wall-flower ([Pl. I.] fig. 54), the foramen in the figure being indicated by a *; it will be noticed also that the funiculus runs down one side of the ovule, so as to terminate at the bottom or base of the nucleus. In ripe seeds, the spot at which the funiculus has been attached is mostly perceptible in the form of a scar. The slight prominence of the foramen can also often be distinguished, as in the seed of chickweed ([Pl. I.] fig. 51*); in the ripe seed the foramen is termed the mícropyle, and towards it the radicle of the embryo is always directed.
One of the cells of the nucleus near its apex then enlarges, so as to form a sac, called the embryo-sac. This is excessively thin and transparent ([Pl. I.] figs. 45 b & 47); and in it, also at the end next the foramen, one or more (in the chickweed one) smaller cells are formed from the cell-contents of the embryo-sac, which are called the embryonal vesicles ([Pl. I.] fig. 45 a).
Thus far developed, the embryo exists prior to the expansion of the flower and the discharge of the pollen. The embryo-sac is not figured in this early condition, the embryonal vesicle being then smaller than that in fig. 45 b, although occupying the same position.
When the pollen has escaped from the anthers and fallen upon the stigma, the pollen-tubes growing down the intercellular passages of the style, enter the foramen of the ovule, and so reach the apex of the nucleus, at which the embryonal vesicle contained in the embryo-sac is situated. The end of the pollen-tube then adheres to the embryonal vesicle, and such interchange of cell-contents takes place between them as effects fertilization.
The process of cell-formation in the fertilized embryonal vesicle then takes place rapidly, new cells being formed by the division of its cell-contents ([Pl. I.] fig. 45 a); and it will be noticed that the new cells are formed at the end of the embryonal vesicle, opposite to that situated at the apex of the embryo-sac. As the cell-division and formation proceed further, a mass of new cells is produced ([Pl. I.] figs. 46 c & 47), forming the rudimentary embryo; and from this, by further growth, the perfected embryo (fig. 55) results; or, to use a fashionable technical term, the simply cellular embryonic mass becomes differentiated into the radicle, cotyledons, and plumule, forming the embryo. It will be remarked that the position of the embryo in fig. 55 is the reverse of that in figs. 46 & 47, the radicle in the former being directed downwards, whilst that of the embryo in the figure of the embryo-sac (fig. 47) is directed upwards.
The embryonal structures are very difficult of detection; but it happens that in our little chickweed they are more easily dissected out than in most other plants. For this purpose, the ovules, placed on a slide and lying in water, should be picked to pieces with the mounted needles, under the simple microscope. They may be preserved in chloride of calcium or glycerine.
A clear distinction must be drawn between seeds, which result from the process of fertilization, and buds, which are formed independently of this process. Both consist essentially of embryo plants; but while the former originate from a single cell, the latter are outgrowths of a parent stem, from which their tissues are derived; and while the former propagate the species, the latter increase the individual.