ALTERNATION OF GENERATIONS 683 



groups, dividing the life of each into two separate epochs, in which 

 it presents itself under two very distinct phases that contrast 

 remarkably with each other. Thus, the frond of Marchantia, 

 evolved from the spore and bearing the antherids and archegones, 

 is that which seems naturally to constitute the plant ; but that which 

 represents this phase in the ferns is the minute Marckantia-like 

 prothallium. In ferns, on the other hand, the product into which 

 the fertilised ' embryo-cell ' evolves itself is that which is commonly 

 regarded as the plant ; and this. is represented in the liverworts and 

 mosses by the sporogone alone. 1 * We shall encounter a similar 

 diversity (which has received the inappropriate designation of * alter- 

 nation of generations ' ) in some of the lower forms of the animal 

 kingdom. 



1 For more detailed information on the structure and classification of the Crypto- 

 gams generally the reader is referred to Goebel's Outlines of Classification and 

 Special Morphology and De Bary's Comparative Anatomy of the Phanerogams 

 and Ferns, translations of both of which have been published by the Clarendon 

 Press ; and especially to Bennett and Murray's Handbook of Cryptogamic Botany, 

 published by Longmans (London, 1889). 



