36 GUIDE TO THE STUDY OF LICHENS. 



These brief references to the algae, with the accom- 

 panying illustrations, will suffice at present, as they 

 will again be mentioned in the discussion of lichen- 

 genera. 



III. LICHENS AS MORPHOLOGICAL UNITS. 



We shall now briefly summarize those characteristics 

 which distinguish lichens from other plants, fungi and 

 algae in particular. This is very necessary in the 

 present state of our knowledge of lichens, as the influ- 

 ence of Schwendener's teachings is as yet very strongly 

 felt. 



Lichens macroscopically considered present such a 

 peculiar appearance that the most superficial observer 

 is led to believe that they form a group by themselves. 

 They occur in places where neither alga nor fungus 

 could exist alone. They have wonderful powers of 

 resisting extremes of temperature ; freezing only checks 

 their growth; a temperature of — 40° C. does not kill 

 them. They will resist a temperature of 60° C. for 

 an hour or more. They can tide over periods of dry- 

 ness which would invariably be fatal to either alga or 

 fungus if existing alone. 



The two symbionts form a microcosmos which is en- 

 abled to perform the life-functions originally inherent 

 in both, and, in addition, the lichen has acquired new 

 structural and functional characters during its phyl- 

 ogeny. The morphological adaptations are primarily 

 for the furtherance of the function of assimilation, 

 while among fungi the structural adaptations are pri- 

 marily for the furtherance of the function of reproduc- 



