802 ECOLOGY 



Another indication of autonomy is seen in the geographic distribution of 

 lichens. As a class, they are among the most xerophytic and autophytic 

 of plants, many species growing on the driest and barest of rocks, where 

 few other plants can maintain themselves. Yet lichens are made up of 

 algae, which as a group are characteristically hydrophytic, and of fungi, 

 which as a group are characteristically mesophytic and dependent; 

 the symbiotic union of two such diverse elements appears to result in a 

 form widely different from either, and more resistant and independent 

 than is to be found in almost any other group of plants. 



The nature of lichen symbiosis. The parasitism of the fungal sym- 

 biont upon the algal layer is undoubted, but there are various theories 

 concerning the relation of the alga to the fungus. 

 A common theory is that the relationship is one 

 of helotism or slavery, the algal symbiont leing 

 represented as indifferent to the fungus. Another 

 common theory is that of reciprocal parasiiism, 

 FIG. 1117. Analgal allied to which is the recently proposed theory of 

 cefl of a lichen (Cla- endosaprophytism, and also the older view tha. the 



donia furcata), which ^ , jj. within the f g j, ^ better 



is closely embraced by 



hyphal filaments of the protected than when separate and thus are enabled 

 lichen fungus; highly to live in drier habitats. Still another view is that 

 ~~ Fr m the alga is merely the host of an ordinary parasite. 

 The parasitism of the fungus is demonstrated 

 clearly by a number of facts, such as the close embracement of algal 

 cells by fungal hyphae (fig. 1117), by the frequent entrance of hyphae 

 into algal cells, by the occasional development of haustoria, by the 

 disorganization and subsequent emptying of the contents of many 

 algal cells, and, perhaps, by the apparent restriction upon algal activity, 

 which is evidenced by increased vigor when released from symbiosis. 

 As for the algal symbiont, the theory of endosaprophytism on fungal 

 excreta seems most tenable in view of the fact that lichen algae are de- 

 cided mixophytes which thrive particularly on peptones; water and 

 salts must also be derived through the medium of the fungus. 



The origin of lichens. There is almost no experimental know edge 

 concerning the origin of lichens, and most of the common species appear 

 to be well-defined lichens without obvious indications of their evolution. 

 In some instances the fungal symbiont lives saprophytically on bark or 

 on humus or parasitically on the plant which gives it mechanical support, 

 as well as on its algal layer ; such species are most abundant in the tropics 



