436 VEGETABLE PHYSIOLOGY 



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 (Edogonium 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 Coleochcete ; 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 spore which is furnished with cilia. 

 Other complex structures are found as the result of the 

 growth and development set up by fertilisation in the 

 Bhodophycece. These are known as cystocarps, and they have 

 been held to represent the sporophytes of those plants. It 

 is important to notice, however, both in their case and in 

 that of Coleochcete, that only part of the structure in most 

 cases is derived from the contents of the fertilised cells, 

 the rest coming from other cells of the tissue of the game- 

 tophyte. 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 iheca with its stalk is the sporophyte. In the 

 Ferns the sporophyte is the predominant form and takes 



