226 THE EVOLUTION OF PLANTS 



is no reason why they should not be, for both 

 spend their lives in the sea, under precisely the 

 same conditions. 



In a Fern it is different; here the prothallus 

 is tied down to a semi-aquatic mode of life by 

 the necessities of fertilisation, which demand 

 the presence of water; the asexual generation, 

 with its spores dispersed in air, is under no such 

 limitation, and can develop as a thorough-going 

 land-plant, with all the high organisation, vas- 

 cular system, mechanical supporting system and 

 so on, which life in air demands. 



While the asexual individual advanced, the 

 prothallus probably became reduced, for the 

 land-plant had to make the most of its land- 

 phase and to economise on the less adaptable, 

 water-bound prothallus. 



On this theory, then, the sexual prothallus 

 and the asexual plant are both alike derived 

 from a thallus, and may once have been per- 

 fectly similar to each other; the one has gone up 

 and the other down. 



The idea of stem and leaf having originated 

 from the differentiated branches of a thallus is 

 quite familiar. Examples of transitional forms 

 between a thallus and a leafy stem are quite well 

 known among Liverworts, and also occur in some 

 of the higher Seaweeds. These are only analogies, 

 it is true, but they give the thallus-theory a 



