IN SPERMATOPHYTES. Among the seed-plants the conditions are quite different. The fertilized egg which lies within the embryo-sac of the ovule proceeds to germinate at once and to form an embryo of a variable degree of advancement, and then the whole process is checked by the final hardening of the seed coats. On account of this the embryo lies dormant for an indefinite period, and when the seed is said to germinate the embryo renews its activity and escapes. In this way it is convenient definitely to mark the embryo as the plantlet that is developed within the seed. As might be expected, such embryos differ very much in the amount of their develop ment. Some pass into the dormant stage at a very early period in their history, so that they are hardly more than a small indefinite group of cells; others develop several distinct organs, and resemble a well-defined plantlet before pass ing into the dormant stage. Among seed-plants, while there is considerable variation in the de tails of germination, the general sequence is as follows: The fertilized egg develops a trans verse wall, and successive transverse divisions result in a linear row of cells, the cell at one end of the row- being anchored to the Avail of the embryo-sac. The cell at the free or deeper end of the row is the one which either exclusively or chiefly enters into the formation of the embryo, the rest of the row forming the so-called 'sus pensor' (q.v.), which is an organ of the embryo that serves to connect it with the food-supply. In any event, this development of the suspensor results in placing the cell which is to develop the embryo more in the centre of the sae, and hence in a position to be better imbedded in the nutritive tissue.
The monocotyledons and dicotyledons are fun damentally different in the organization of their embryos. In both groups the root-tip is organ ized at that end of the axis which points toward the mieropyle (the opening left by the integu ment), and for this reason it is the root-tip that first emerges from the seed. In the ease
of the monocotyledons the end of the axis remote from the root-tip organizes the single massive seed-leaf (cotyledon), while the stem-tip arises laterally, just behind the cotyledon. Naturally in such an embryo there can be but a single cotyledon, and this fact has given name to the group. In the dicotyledons this remote end forms the stem-tip, while the cotyledons appear as lateral menthers just behind the stem tip. This prevailingly results in two cotyledons. has suggested the name of the group; but it does not preclude the development of a single cotyledon, or of more than two. For example, this type of embryo belongs to the conifers, where in many eases the cotyledons form a rosette of several members. The regions of the complete dico tyledonous embryo, which are more easily defined than those of the monocotyledonous embryo, are as follows: The axis beneath the two cotyledons, at the tip of which is the embryonic root struc ture, has been named the 'hypocotyl."1'his has been variously called `cauliele' and 'radicle,' one name expressing the idea that it is a little stem and the other that it is a little root. It is, however, so peculiar in its structure and powers that it can hardly be called either a distinct stem or a dis tinct root, so that it has received a name of its own, and may be regarded as an organ of the embryo. At the summit of the hypocotyl the two cotyledons occur, variously arranged with ref erence to one another and to the hypocotyl in the seed. Between the cotyledons, as if continuing the axis, there is often a minute bud called the `plinule,' which, after the escape of the em bryo, develops the shoot. For an account of the escape of the elDbry0, see SEED.