PARASITES 



339 



the form of lichens have already been described (Sect. 331). 

 It is not correct to describe the condition of such algae 

 as slavery if the term is meant to imply that they derive 

 no benefit from the association. Perhaps serfdom is a 



-t 



t 



FIG. 236. Roots of Red Clover with Tubercles. 



/, sections of ascending branches ; 6, enlarged base of stem ; t, root-tubercles 

 containing bacteria. 



more suitable word, though it is not the term used by 

 botanists. At all events, the alga is enclosed within a 

 network of fungus hyphae from which it cannot readily 

 escape, and there does most of the work of the lichen, 

 including all of the manufacture of food from carbon 

 dioxide. 



