43 



the ready exchange of gases ; as, for example, the breathing pores 

 and the cyphellae. The breathing pores are the intercellular canals 

 met with in the upper and lower cortical layers, which are continuous 

 with those of the dermis ; they are especially numerous in the 

 thinner portions of the layers. The breathing pores, and especially 

 the cyphellae, will be more fully discussed in another chapter. 



The primary function of this layer is usually mechanical. Two 

 other functions almost equal in importance are those of protecting 

 the underlying tissues, especially the algae, and preventing the sud- 

 den and excessive evaporation of moisture. That the primary func- 

 tion is purely mechanical in most cases, is evident from its structural 

 adaptations in form and position to meet mechanical requirements. 

 In some lichens it is likewise evident that the primary function is not 

 mechanical, since we find special mechanical tissues distinct from the 

 cortical layer, as, for example, in the podetia of Cladonia and in 

 some species of Gvrop/wra, as well as in many of the fruticose thalli. 

 In these cases the prime function is evidently to protect the under- 

 lying algae. 



(c) The Algal Layer. (Gonidial Layer.} 



This is by far the most important structure of the thallus. It con- 

 tains the symbiotic algae whose special function is that of assimila- 

 tion. The layer consists of hyphal filaments continuous with the 

 cells of the cortical and medullary layers, and the algal cells which 

 are in more or less intimate union with the terminal branches of the 

 hyphae ; it varies considerably in thickness ; in the same species it is 

 thickest in the young growing plant ; in the older portions of the 

 thallus the algae are less numerous. As already indicated, the as- 

 similating algal layer makes suitable concessions as to position to 

 the mechanical tissue. As a rule, the algae are most numerous where 

 the cortical layer is thinnest. 



The subject of greatest interest is the relation of the algae to the hy- 

 phae. It will be remembered from the historical review that Schwen- 

 dener, in 1863, and others at one time, believed that the algae 

 (gonidia) develop from terminal branches of the hyphal filaments. 

 From this it is evident that they recognized the close organic union. 

 It is, however, also evident that this association is not very intimate 

 in many forms of lichens, for instance in the Collemaceae, in which 

 the algal symbiont is represented by the genus Nostoc. We find the 



