EEPEODUCTION 427 



as far as the Thallophytes are concerned. There are indi- 

 cations of its origination in that group, but they are ex- 

 tremely rudimentary, and occur in families which are widely 

 separated from each other. The gametophyte was doubtless 

 the primitive form of the plant, and in some way or other 

 the sporophyte took its origin from it. Certain phenomena 

 which may represent stages in the process can still be 

 observed. In CEdogonium the fertilised cell does not grow 

 out into a new filament, but produces in its interior four 

 zoospores which escape from it, and after a period of rest 

 germinate and produce new plants. The fertilised cell here 

 may perhaps represent the sporophyte, reduced however to 

 a single sporangium. An even simpler stage of develop- 

 ment may perhaps be recognised in Spirogyra, where the 

 nucleus of the fertilised cell divides into four, though no 

 definite cells are formed. On germination of the zygote 

 however only one filament grows out. A more complex 

 structure is formed in Coleoclicete ; the zygote becomes 

 invested with a covering derived from the adjacent cells, 

 and after sinking to the bottom of the water, it germinates, 

 producing inside its coating a small mass of cells, each one 

 of which liberates a zoospore. Other complex structures 

 are found as the result of the growth and development set 

 up by fertilisation in the Ehodophycese. These are known 

 as cystocarps, and they have been held to represent the 

 sporophytes of those plants. It is important to notice how- 

 ever, both in their case and in that of Coleochaete, that only 

 part of the structure in most cases is derived from the con- 

 tents of the fertilised cells, the rest coming from other 

 cells of the tissue of the gametophyte. As we have seen 

 the sporophyte in the higher plants is entirely derived from 

 the zygote. 



The antithetic alternation of generations is seen most 

 clearly in the groups of the Mosses and Ferns. In the 

 former the Moss plant is the gametophyte, the so-called 

 capsule or theca with its stalk is the sporophyte. In the 

 Ferns the sporophyte is the predominant form and takes 



