632 LECTURE XXII. 



of generations can be traced since there is no certainty as 

 to the nature of the form to which any given spore may 

 give rise: the individual developed from the asexually pro- 

 duced spore is not, as in a life-history of the Moss-type, 

 necessarily sexual, nor is the individual developed from the 

 sexually produced spore necessarily asexual. In others a 

 more or less regular alternation of generations is traceable. 

 Thus in Acetabularia (Siphoneae), the plant produces resting- 

 spores asexually, which, as already mentioned, behave as 

 gametangia; the gametes conjugate to form a zygospore, and 

 from the zygospore the Acetabularia springs. Here the 

 alternation of generations is quite regular. The Acetabularia- 

 plant is the asexual generation or sporophore ; the resting- 

 spore alone represents the sexual generation, or oophore, 

 inasmuch as it directly gives rise to sexual reproductive 

 cells. The life-history of Botrydium is essentially the same 

 as that of Acetabularia, but it is frequently less regular; thus 

 the Botrydium-plant may produce, instead of resting-spores, 

 uniciliate zoospores by which it is directly reproduced, and in 

 this way several asexual generations may succeed each other. 

 The fact that the oophore may be actually asexual, as when 

 the resting-spore developes directly into a Botrydium-plant, 

 or when, as mentioned already, it produces zoospores instead 

 of gametes, does not affect the alternation of generations; 

 the oophore is present and from it the sporophore is derived ; 

 the asexual oophore may be conveniently distinguished as 

 a potential oophore. In Coleochaete we have a case in which 

 the normal alternation of generations is interfered with by 

 the asexuality of several successive generations of what ought 

 to be sexual forms. The sexual individual produces the 

 oospore, and the oospore gives rise to a small individual 

 which is asexual, and which produces zoospores; from these 

 zoospores are developed individuals which resemble the 

 sexual form in all respects save that they do not produce 

 sexual organs, but produce only zoospores. At length, after 

 a series of generations, a sexual plant is developed. The 

 appearance of the sexual plant seems to be determined 

 by the season of the year. The oospores germinate in the 



