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THE CONTINUITY OF THE RACE 



Fig. 17.15. Alternation of generations in the Spermatophyta. The life history of a flowering 

 plant, the lily. The male gametophyte (pollen grain) develops from a microspore in the 

 anther; the female gametophyte develops from a megaspore in the ovule within the ovary 

 A, the mature sporophyte generation. B, the flower. C, a cross section through the anther, 

 showing pollen grains. D, a pollen grain showing the tube nucleus and generative nucleus. In 

 the lily the generative nucleus does not divide until after pollination occurs. E, the germinat- 

 ing pollen grain, with pollen tube containing the tube nucleus and two sperm nuclei formed 

 by division of the generative nucleus. The pollen tube grows down through the style and 

 ovary wall to the opening of the ovule. F, cross section of ovary showing ovules, two to 

 each of the three carpels making up the stigma in this plant. G, the mature seven-celled 

 female gametophyte, with egg (ovum). H, fertilization of the egg by one of the two sperm 

 nuclei, and fusion of the other with the endosperm nuclei. /, development of the embryo 

 from the egg, and of the endosperm from the primary triploid endosperm cell, within the 

 ovule. J, the germinating seed. {Modified from Turtox chart, courtesy General Biological 

 Supply House, Inc.) 



these, the asexual megaspore, receives nearly all the cytoplasm, and the 

 other three degenerate (like the polar bodies in animal oogenesis). Micro- 

 spore and megaspore formation thus correspond closely to spermatogenesis 

 and oogenesis in animals, with the difference that the spermatozoa and 

 eggs of animals unite directly to produce the diploid zygote, while the 



