218 Plant Life and Evolution 



the form and character of a distinct organism. 

 There is no doubt that the fungus element is para- 

 sitic upon the alga, upon which it is dependent for 

 its existence, but the alga seems to suffer little from 

 its imprisonment in the tissues of the fungus, and as 

 the latter takes up water very quickly and retains it 

 tenaciously, the alga is undoubtedly enabled to grow, 

 thanks to the shelter of the fungus, where otherwise 

 it could not exist. Moreover, the recent evidence 

 that some of the higher fungi, as well as bacteria, 

 can assimilate nitrogen, makes it quite probable that 

 the fungus gives to the alga certain nitrogenous 

 compounds in return for the carbonaceous food 

 taken from it. It is likely that in the case of the 

 association of the fungus with a saprophytic flow- 

 ering plant, nitrogen is also furnished to the host as 

 well as carbon. 



The symbiotic association of two green plants is 

 much less common, but a good many cases are 

 known. Usually one of the symbionts is a blue- 

 green alga, and it is possible that here also there 

 may be a case of nitrogen assimilation which may 

 be useful to the other party of the association. A 

 number of liverworts, e.g., Blasia, Anthoceros, 

 have always associated with them a species of 

 Nostoc, and the little water-fern, Azolla, always 

 harbors in its leaves colonies of a blue-green alga 

 (Anabaena). No actual parasitism has been shown 

 in any of these cases, and just what the relation of 

 the two symbionts is we really do not know. 



