770 



PART IV. — 'I HE PHYSIOLOGY OF PLANTS. 



cells (gametes), whilst others remained simply zoospores ; so that 

 the plant becomes a gametophyte still possessing the capacity of 

 i-eproducing itself asexually by zoogonidia, though in some forms 

 (Conjugatae, Fucaceee, Charoideag) this capacity has entirely 

 disappeared. 



The evolution of the entirely asexual sporophyte is to be traced 

 to the advantage gained by the development of more than a 

 single organism from the sexually-produced spore, that is, hj poli/- 

 embryony. Thus in Yaucheria and Chara the oospore gives rise 

 directly to a single sexual individual, and the oospore is all that 

 represents the sporophyte in the life-history of these Algse ; and 

 ■this applies also to Fucus, but here a multiplication of individuals 

 is attained by the development of eight oospheres in the oogonium. 

 A rudimentary form of polyembryony is presented by the Desmids 

 where the zygospore gives rise to two individuals. It is more 

 marked in such plants as Sphaeroplea and CEdogonium, where the 

 oospore gives rise to several (2-8) zoospores from each of which 

 a new sexual individual is developed : here the oospore and its 

 zoospores represent the sporophyte. The sporophyte first makes 



fitfi^y^ppearance as a distinct cellular organism in Coleocheete 

 (p. 250), and from this point onwards its further evolution can 

 jbe readily traced through the sporogonium of the Bryophyta to 

 the Pteridophyta and Phanerogamia where it is the predominant 

 form in the life-history. The sporophyte is thus essentially o^mo- j 

 g^nic, that is, it is developed as the result of a sexual process 

 (p. 69), 



Whilst in the majority of cases the pporophyte is developed from a sexually- 

 produced spore, it is sometimes developed from the fertilised female organ ; 

 this is the case in plants in which there are either no differentiated gametes 

 {e.g. Eremai-cus, p. 297), or no differentiated female gamete {e.g. Khodophyceae, 

 p. 269) ; but it is still the product of a sexual process. In some cases of 

 sexual degeneration {e.g. apogamous Ferns, p. 378 ; Ascomycetes, p. 298) the 

 sporophyte is developed vegetatively (p. 87). 



The sexual formation of spores is the result of the fusion of 

 two sexual reproductive cells or gametes, neither of which is, as a 

 rule, capable by itself of developing into a new individual ; 

 though, in some of the lower plants in which sexual differentia- 

 tion is incomplete (e.g. Ulothrix, Ectocarpus; see p. 226) the 

 gametes may germinate independently. The act of fusion is a 

 sexual process, and the result is a cell (oospore or zygospore, p. 

 80) which proves itself to be a spore by developing into a new 



