ALTERNATION OF GENERATIONS 147 



conditions under which the plants were cultivated. 

 Over and above this development of sexual organs and 

 of gonidia, however, it had been found that in some 

 Thallophyta another mode of multiplication had made 

 its appearance. In Oedogonium, for instance, the 

 fertihsed ovum did not become a new plant directly 

 but subdivided into four motile " zoospores," each of 

 which became in due course a new Oedogonium. It was 

 claimed by some authorities, notably by Scott, in 1896, 

 that " the zoospores formed from the oospore on germina- 

 tion are identical with the so-called zoogonidia formed 

 on the vegetative plant at all stages of its growth," and 

 holding this view, Scott, and others who agreed with 

 him, rejected the theory that suggested that such a 

 subdivision of the oosperm indicated the first beginnings 

 of what was destined to become the Bryophytic sporocarp. 

 The two theories may be very briefly summarised and 

 contrasted by saying that in the antithetic theory of 

 Celakowski and Bower the essential feature is " the 

 recognition of the spore - producing generation as an 

 interpolated phase in the life history . . . and not as 

 derived by the modification of a generation resembling 

 the sexual one/' while according to the homologous 

 theory " the spore-producing generation in Bryophyta 

 and Pteridophyta is strictly homologous with the sexual 

 generation and the alternation in these groups differs 

 only in its regularity from the succession of sexual and 

 asexual forms in Thallophyta." 



The problem was compHcated still further by the 

 discovery of a remarkable cytological difference between 

 the gametophyte and sporophyte. It had been known 

 for some time that nuclei in the course of their division 

 always exhibited a definite number of chromosomes 

 pecuhar to the cells of the specific plant under considera- 

 tion. In 1893 Overton found that the nuclei of the cells 

 of the prothallus of Ceratozamia, one of the Cycadaceae, 

 had only half the number of chromosomes present in the 



