212 



CYTOLOGY CHAP. 



and diploid states than do animals. In the latter — at least in the 

 Metazoa, with certain rare exceptions such as the males of the Hymenoptera 

 — the individual is always diploid. In each life cycle occur one (post- 

 reduction) or probably two (pre-reduction) haploid cell generations, 

 namely, the gamete itself and generally the secondary oocyte or sper- 

 matocyte. In plants, however, the processes of meiosis and syngamy are 

 often separated by a long section of the life history giving rise to an 

 alternation of haploid and diploid generations. One of the best illus- 

 trations of such an alternation is in the ferns, where, as is well known, 

 the ordinary fern plant or sporophyte is diploid, and the prothallus or 

 gametophyte is haploid. The sporophyte produces, with reduction of 

 chromosomes, haploid spores from which grows the prothallus. This 

 produces gametes, without of course any further reduction of chromosomes, 

 and from the zygote cell develops the next sporophyte. In the fern 

 therefore the dominant phase in the hfe history is the diploid generation. 

 This is still more so in the case of the flowering plants, where the haploid 

 generation or gametophyte is reduced to a very few cell generations 

 (5 in the female and 4 in the male), and does not lead an independent 

 life, but is borne on and nourished by the sporophyte, which is the 

 plant body as we know it. 



The cell generations involved in the haploid phase of the flowering 

 plants are shown diagrammatically in Fig. 90. The diagrams start with 

 the pollen mother-cell in the male and the embryo-sac mother-cell in the 

 female — in each case the last cell generation of the diploid phase. These 

 cells divide twice in rapid succession, giving rise each to a group of four 

 cells. Reduction takes place in the first of these two divisions. The 

 process therefore is closely parallel to the meiotic phase in animals. 

 The four haploid cells formed by these two divisions are spores, homologous 

 with the spores of ferns, mosses, etc. In the male, the spores are micro- 

 spores or pollen-grains. The nucleus of the pollen grain divides into 

 two, one being a vegetative nucleus and the other a nucleus which again 

 divides to give two gamete nuclei. 



In the female, typically only one out of the four spores {megaspores) 

 derived from a single embryo-sac mother-cell develops, the other three 

 degenerating and thus again reminding us very strikingly of the ovum 

 and polar bodies in animals. The nucleus of that megaspore which is 

 destined to proceed with its development divides three times, thus pro- 

 ducing eight nuclei. Since cell division does not foUow these mitoses, 

 these nuclei are all contained in one large vacuolated ceU, the embryo- 

 sac. Of the eight nuclei, only one is a functional gamete nucleus (ovum) ; 

 three of the remainder become the antipodal nuclei, two the synergidae, 

 and the remaining two come together in the middle of the cell and fuse 

 to form the central fusion nucleus, which is therefore diploid. 



