436
ANGIOSPERMS
nucellus, so that it projects into or beyond themicropyle. By this time the two male gametes have passed the degenerated vegetative energid and are at the apex of the tube, and this now being softened they escape into the plasm. of the embryo-sac. The gametes, as observed in recent investigations, have vermiform nuclei somewhat thickened at the end, and exhibit a finely porose structure; they are not unlike the spermatozoids of some Pteridophytes, and they are sometimes so named, but their independent movement, without or by means of cilia, has not been observed. One of the male gametes thus discharged, reaches the egg and impregnates it. The fate of the other was until tio*UiZa' recently not traced, and it was supposed to -disintegrate in the plasm of the embryo-sac, or occasionally to coalesce with synergidae. We now know, however, that in several plants it passes downwards, perhaps passively transported by the streaming of the protoplasm in the embryo-sac, to the two polar energids which have not yet coalesced, and it unites with the apical polar one, or it may await their coalescence, and then unite with their combination—the definite energid of the embryo - sac. The fusion of the second male gamete apparently precedes the impregnation of the egg by the first male gamete. This remarkable double fertilization as it has been called, although only recently discovered, has been proved to take place in widely separated families, and both in Monocotyledones and Dicotyledones, and there is every probability that, perhaps with variations, it is the normal process in Angiosperms. It gives a history of the second male gamete. Whilst the pollen-tube normally reaches the apex of the embryo-sac through the micropyle (acrogamy or porogamy), it may pierce the embryo-sac at the chalazal end or at the side (basigamy or chalazogamy) by entering the base of the nucellus and making its way upwards, using as passages, it may be, the cavities of aborting embryo-sacs. After impregnation the fertilized egg segments to form the embryo, the synergidae having long before disintegrated, and the impregnated definite energid also enters sooner or later upon division which results in the formation of endosperm; the antipodal cells often disintegrate at the same time, though they may undergo multiplication first. It has long been known, that after fertilization of the egg has taken place the formation of endosperm begins from the definite energid, and this had come to be regarded as the recommencement of the development of prothallus after a pause following the reinvigorating union of the polar energids. This view is still maintained by those who differentiate the two fertilizations within the embryo-sac, and regard that of the egg by the first male gamete, as the true or generative fertilization, and that of the polar energids by the second male gamete as a vegetative fertilization which gives a stimulus to development in correlation with the other. If, on the other hand, the endosperm is the product of an act of fertilization as definite as that giving rise to the embryo itself, we have to recognize that twin plants are produced within the embryo-sac—one, the embryo, which becomes the angiospermous plant, the other, the endosperm, a short-lived undifferentiated nurse to assist in the nutrition of the former, even as the subsidiary embryos in a pluriembryonic Gymnosperm may facilitate the nutrition of the dominant one. If this is so, and the endosperm like the embryo is normally the product of a sexual act, hybridization will give a hybrid endosperm as it does a hybrid embryo, and herein (it is suggested) we may have the explanation of the phenomenon of xenia observed in the mixed endosperms of hybrid races of maize and other plants regarding which it has only been possible hitherto to assert that they were indications of the extension of the influence of the pollen beyond the
egg and its product. This would not, however, explain the formation of fruits intermediate in size and colour between those of crossed parents. The signification of the coalescence of the polar energids is not explained by these new facts, but it is noteworthy that the second male gamete is said to unite sometimes with the apical polar energid, the sister of the egg, before the union of this with the basal polar one. The idea of the endosperm as a second subsidiary plant is no new one; it was suggested long ago in explanation of the coalescence of the polar energids, but it was then based on the assumption that these energids were male and female gametes, for which there was no evidence, and which was inherently improbable. The proof of a coalescence of the second male gamete with the definite energid gives the conception a more stable basis. By the segmentation of the fertilized egg, now invested by cell-membrane, the embryo-plant arises. A varying number of transverse segment-walls transform it ° into a pro-embryo—a cellular row of which the geay, micropylar cell becomes attached to the apex of the embryo-sac, and thus fixes the position of the developing embryo, and the terminal cell is projected into its cavity. In Dicotyledones the shoot of the embryo is wholly derived from the terminal cell of the pro-embryo, from the next cell the root arises, and the remaining ones form the suspensor. In many Monocotyledones the terminal cell appears to form the cotyledonary portion alone of the shoot of the embryo, its axial part and the root being derived from the adjacent cell; the cotyledon would thus be a terminal structure and the apex of the primary stem a lateral one—a condition in marked contrast with that of the Dicotyledones. It is known, however, that in many Monocotyledones the cotyledon is not really terminal. The primary root of the embryo in all Angiosperms points towards the micropyle. The developing embryo at the end of the suspensor grows out to a varying extent into the forming endosperm, from which by surface absorption it derives plastic material for growth; at the same time the suspensor probably plays a direct part as a carrier of nutrition and may even develop, where perhaps no endosperm is formed, special absorptive “ suspensor roots ” which invest the developing embryo or pass out into the body and coats of the ovule, or even into the placenta. In some cases the embryo or the embryosac sends out haustoria into the nucellus and ovular integument. As the embryo develops it may absorb all the plastic material available, and store either in its cotyledons or in its hypocotyl what is not immediately required for growth as reserve-food for use in germination, and by so doing it increases in size until it may fill entirely the embryo-sac; or its absorptive power at this stage may be limited to what is necessary for growth and it remains of relatively small size, occupying but a small area of the embryo-sac which is otherwise filled with endosperm in which the reserve-food is stored. There are also intermediate states. The position of the embryo in relation to the endosperm varies, sometimes it is internal, sometimes external, but the significance of this has not yet been established. Apogamy sometimes occurs in Angiosperms. As a rule the embryonic shoots are developed from the nucellar wall, or from the synergidm, or from the antipodal cells, and polyembryony is the result. In Erythronium, however, polyembryony is the result of a process recalling features of Gymnosperms, for the embryos arise from the segmentation of the terminal cell of the suspensor—that which normally produces the one embryo. True parthenogenetic apogamy is said to occur in Antennaria alpina. The formation of endosperm starts, as has been stated,
