Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/446

Rh 428 REPRODUCTION [VEGETABLE. oospheres of the Saprolegnieae. In this case that differen- tiation of the protoplasm of the oogonium into ooplasm and peripiasm which has been described above as occurring in the closely allied and completely sexual Peronosporeae does not take place, but the whole is used in the formation of the oospheres. It is quite clear also that in the Sapro- legnieae no sexual process takes place. In some species of Saprolegnia the antheridia are altogether absent, in others they are rudimentary, and even in those Saprolegnieae (some species of Saprolegnia, Achlya, Aphanomyces) in which the antheridia are well-developed they remained closed. No case is known of male parthenogenesis, that is, of the development of an individual from a male repro- ductive cell, among plants in which the sexual differentia- tion of the reproductive cells is well-marked, but there are instances of the kind in more lowly organized plants. Thus in the Mucorini, mentioned above, one of the azygo- spores produced may be regarded as belonging to a male organ ; again, in Ectocarpus the male planogamete germi- nates independently as well as the female. Among plants the sexual organs of which do not produce specialized re- productive cells instances of parthenogenesis also occur. Thus the spores (ascospores) which are produced by the female organ (ascogonium) of those Ascomycetes, such as the Erysiphese, Penicillium, Melanospora, Xylaria, in which the male organ is f unctionless or absent can only be regarded as being parthenogenetically produced. Admitting, then, that sexual differ from asexual reproductive cells in the lack of something which the latter possess, and which was thrown off either from the former or from the organs which produce them, we may go on to inquire what this something may be. Our in- formation on this point is very scanty, but Strasburger's ise of views throw some light upon it. He considers, namely, uality. that the formative processes of the cell are regulated by the hyaline plasma of the nucleus the nucleo-idioplasma, as he terms it ; the richer the nucleus is in this substance the more capable is the cell of producing new cells. The asexual reproductive cells are then cells the nuclei of which are especially rich in this substance. He considers that the differences between asexual and sexual reproductive cells is quantitative and not qualitative that the nuclei of the former are rich in nucleo-idioplasma, those of the latter poor, either originally or by the throwing off of part of their substance in the mode described above. In en- deavouring to account for the further differentiation of sexual reproductive cells into male and female, it seems natural to suggest that the respective nuclei have under- gone a qualitative differentiation, and that in the sexual process the qualitative, as well as the quantitative, incompleteness of each is made good. Strasburger is, however, strongly of opinion that this is not so, but that the difference is purely quantitative. But it must be pointed out that, according to this view, there is no reason why a sexual process should not take place, either between two male cells or organs, or between two female cells or organs, a possibility which is never realized, nor is it possible to account for the fact to be subsequently dis- cussed that in many cases a sexual process cannot take place between sexual cells or organs of a closely-allied origin. The nature of the sexual process will be better under- stood by a detailed description of it in some particular case. The following is a brief account of the results of Strasburger's observations on the process of fertilization in Angiosperms. xual At the period of the dehiscence of the anther, the protoplasm of ocess the pollen-grain undergoes division into two cells a smaller, the Angio- generative cell, and a larger, the vegetative cell. At the time of errns. pollination, when the pollen-grain is mature, it is usually the case that the only persisting indication of the previous cell-division is the presence of two nuclei in the protoplasm. In many cases the two nuclei were found to differ qualitatively, inasmuch as the generative nucleus stained readily when treated with carmine, picro-carmine, methyl-green, &c., whereas the vegetative nucleus stained imperfectly or not at all. On the formation of the pollen- tube, the generative nucleus, and sometimes the vegetative nucleus also, is carried down into it, and the former then undergoes division into two ; occasionally one of the new nuclei divides again. The pollen-tube grows down the style, enters the ovary, and is directed to the micropyle of an ovule. In the ovule the egg-apparatus lies at the micropylar end of the embryo-sac, and the delicate wall of the embryo-sac covering it undergoes absorption. In its further growth the pollen-tube comes into contact with the synergidaj ; in some cases its growth now stops, and a portion of its protoplasm, with one or both of the generative nuclei, passes through the mucilaginous wall at the apex of the tube, without leaving any opening behind it, and travels to the oosphere ; in other cases the pollen-tube grows between the synergidre to the oosphere, and at once pours a portion of its protoplasm with a generative nucleus into it. This is followed by the appearance of a second nucleus in the oosphere (the male pronudeus}, which is the generative nucleus derived from the pollen-tube, which now travels towards the nucleus of the oosphere (female pronucleus) and fuses with it ; when the two pronuclei have each a nucleolus these also eventually fuse. Fertilization is now complete. The synergidte take no direct share in the process of fertilization, but become disorganized ; their disorganization usually begins when the pollen-tube first comes into contact with them. They serve merely to direct the pollen-tube or its escaped contents to the oosphere and to provide nourishment for them. The fusion of the male and female pronuclei has been observed also in the oosphere of the Gymnosperms, and in the conjugation of Spirogyra. Since plants are so commonly able to reproduce vege- Signifi tatively, the question arises as to the necessity of the production, either sexually or asexually, of spores. The true re ductio biological importance of these cells is very great. They are capable, namely, of retaining their vitality under external conditions, such as long drought, absence of food, extremes of heat and cold, which would prove fatal to the plant, and they therefore are essential to the maintenance of the species. Further they are of importance in the distribution of the species; they are light, readily transportable by wind or water, in some cases themselves actively motile. In this way they serve to prevent that close aggregation of individuals of the same species which would result from a continued vegetative multiplication, and would prove in- jurious to the species. In the case of Phanerogams, in which the macrospore is not set free from its sporangium, the same ends are obtained by the production of seeds. The further question now arises as to the importance of the sexual process. It appears that, if any given species, at least among Impo: the higher plants, is reproduced through a long series of ance ' generations in a non-sexual manner, the individuals tend to degenerate, and the original well-developed form can only be reattained by the formation of a sexually produced spore. This result is to some extent realized by the fusion of two sexual cells or organs belonging to the same indi- vidual that is, by self-fertilization but more completely when the fusion takes place between sexual cells or organs belonging to distinct individuals that is, by cross-fertiliza- tion. In some plants, as in the Peronosporeae and in those Cross sexual Ascomycetes which have pollinodial antheridia, self- tiuza fertilization alone is possible ; this is also the case in certain Phanerogams in which the arrangements are such (notably in cleistogamous flowers) that only pollen from its own ! anthers can reach the stigma of the flower. In most cases, however, the conditions under which the sexual process is carried on, such as the formation of free -swimming gametes and antherozoids, and of spermatia and pollen-grains which can be readily conveyed from place to place, are such as to favour the occurrence of cross-fertilization. In some there Dicec are special arrangements for the attainment of this end, the most general of which is dioecism, that is, the produc- tion of the male and female organs by distinct organisms. Thus in certain Fucacese (Fucus vesiculosus, F. nodosus,