ALTERNATIONS OF GENERATIONS 551 



which is heterosporous (p. 514). Tl me condition is also in 



certain Ferns (Marsilia, .-holla), in many fossil Lycopods, and 



occasionally in Equiseta also. But still the megaspore in th< 1 plants 

 is shed from the parent before fertilisation, and is then ndent on 



its own resources. The longer the period of connection with the 

 parent the better. A further advantage was then gained by retention 

 of the megaspore itself upon the parent plant until the embryo is far 

 advanced. The sporangium which thus retains its megaspore is called 

 an ovule, and this matures into the seed, which is characteristic of all 

 the Higher Plants of the Land (p. 521). The prevalence of the Seed- 

 habit is the token of its success. 



While a certainty of protection of the prothallus and of continued 

 nutrition of the embryo is thus secured by retention of the mega- 

 spore, or embryo-sac, upon the parent, the steps of progress involved 

 have reacted adversely upon the gametophyte generation. The 

 separation of the sexes tended to relieve it of the necessity for self- 

 nutrition. Provided the microspores are numerous, and each has a 

 sufficiency of material to form an antheridium and spermatozoids, 

 that would meet the requirements, and little or no vegetative tissue 

 is needed. This condition is characteristic of heterosporous plants. 

 It is seen in Selaginella (p. 517, Fig. 41 1), and in the pollen of Gymno- 

 sperms and Angiosperms with their vestigial male prothalli (Figs. 222, 

 421). On the other hand, the megaspore requires to be well nourished, 

 in the interest of the archegonia, and of the embryo which each will 

 bear. But it receives its supply from the parent plant, rather than by 

 its own self-nutrition. Thus in the case of a megaspore of Selaginella 

 the female prothallus is little more than a storage tissue, and a basis for 

 archegonia (Fig. 412, iv.). Self-nutrition is reduced, or entirely absent 

 as in the Gymnosperms ; and, finally, in the Angiosperms the female 

 gametophyte is so transformed that it may be difficult to homologise 

 the contents of the embryo-sac at all with a female prothallus (Fig. 216). 



The spermatozoid motile in water remained, however, as the me. 

 of fertilisation even after the adoption of the Seed-habit. It is still 

 seen in the Ginkgoaceae and the Cycads, though its unpractical nature 

 is evident (p. 527). The last step of emancipation from the original 

 aquatic method of propagation was the substitution of the motile 

 spermatozoid by the non-motile gamete delivered by the pollen-tul 

 (Figs. 226, 424). Thus the most critical point of each life-cyi le, viz. 

 fertilisation, was finally adapted to Life on Land. And so by a 

 series of steps the gametophyte is reduced, altered, and in some ca» 

 almost obliterated. It has paid the penalty of its inability to adapt 



