180 



GENERAL BIOLOGY 



FIG. 66. Diagrammatic Summary of the Life-cycle of a Seed Plant 

 (Alisma). A-G; A, M, N; X, Y, Z, A, the sporophyte phase; H-X ; 

 PX, the gametophyte phase. A, the mature plant whose flowers 

 bear stamens (B, C) and pistils (M ) ; N, a section through the mega- 

 sporangium ; 0R, the developing megaspore ; Q, with the degenerating 

 tapetal (nourishing) cell. There is but one megaspore in Alisma instead 

 of the four that are found in most seed plants. T, the complete embryo- 

 sac with the two endosperm nuclei in the middle, the three antipodal 

 nuclei at the lower end, and the egg-nucleus with the two synergids at the 

 other end ; V, the egg-nucleus. D, the development of the pollen- 

 mother-cells ; E, one of the pollen-mother-cells that divides into F, a 

 tetrad ; G, a microspore (pollen grain) ; H, division of the microspore 

 nucleus ; 7, beginning of the pollen-tube, with one tube-nucleus and the 

 two gametic nuclei ; W, fusion of male pronucleus with egg-pronucleus ; 

 X, the zygote (gametospore) ; Y, beginning of embryo formation ; Z, the 

 seed, containing the developed embryo which grows into A. In some 

 forms the second male pronucleus JP fuses with the endosperm nuclei 

 (L). (After Schailner.) 



