486 BOTANY OF THE LIVING PLANT 



function of producing spermatozoids. It may therefore, and it does, 

 remain small. But the female prothallus has both to produce ova and 

 to nourish the embryo after fertilisation. This can best be carried out 

 by a large prothallus, which will develop better from a well-nourished 

 spore. This is the physiological rationale of the origin of the mega- 

 spore as distinct from the microspore, as seen in Selaginella, which is 

 heterosporous (p. 320). The same condition is also seen in certain 

 Ferns (Marsilia, Azolla), and in many fossil Lycopods and Equiseta. 

 But still the megaspore in these plants is shed from the parent before 

 fertilisation, and is then dependent 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. 283). 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. 321, Fig. 264), and in the pollen of Gymno- 

 sperms and Angiosperms with their vestigial male prothalli (Figs. 194, 

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

 in the interest of the archegonia, and the embryo which it is to 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. 265). Self-nutrition is reduced, or entirely absent 

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

 gametophyte is so transformed that it is difficult to homologise the 

 contents of the embryo-sac at all with a female prothallus. 



The spermatozoid motile in water remained, however, as the means 

 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. 315). The last step of emancipation from the original 

 aquatic method of propagation was the substitution of the motile 



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