VII 



MEIOTIC PHASE IN PLANTS 



213 



At fertilization the two male gamete nuclei which are formed in each 

 pollen grain are both introduced into the embryo-sac ; one fuses with 

 the ovum nucleus to produce the zygote, while the other fuses with the 

 central fusion nucleus, forming thus a triploid nucleus. This afterwards 

 gives rise to the endosperm, or reserve food material of the seed. 



For a more detailed account of the gametophyte and fertilization 

 in the flowering plants the reader is referred to any comprehensive 

 work on botany, such as that of Bower (1919). 



Pollen Mother Cell 



Embryo-sac Mother Cell 



Microspores 

 (Pollen-grains), 



Megaspores 



FIG. 90. 



Diagrams of the cell generations involved in the haploid phase of the flowering plants. 



Mosses and liverworts present the reverse case to the flowering 

 plants, for the dominant phase (the ordinary moss plant, etc.) is the 

 haploid gametophyte. The zygote grows into a comparatively simple 

 sporophyte which is retained on and nourished by the gametophyte, 

 and produces spores with reduction of chromosomes. These are set free 

 to produce the new generation of gametophytes. 



The relations of the haploid and diploid phases of the life cycle in 

 animals and plants is summarized in Fig. 91 ; the figures A-F form a 

 progressive series in the rise of the diploid and reduction of the haploid 



