38 



ualism, as met with among lichens, and mutualism. These explana- 

 tory statements will be more fully discussed in other chapters. 



In their development lichens stand alone. They occur in places 

 where neither alga nor fungus could exist independently. Jumelle 

 has proven experimentally that they can resist much greater ex- 

 tremes of heat and cold than other chlorophyll-bearing plants. Every 

 observer of nature is familiar with their wide distribution and wonder- 

 ful power of adaptability. They are of slow growth and are en- 

 dowed with an exceedingly long life period. The symbionts unite to 

 form a microcosm w r hich not only performs the life functions 

 originally inherent in both, but also additional life functions which it 

 has acquired during its phylogeny as a lichen. The fungal symbiont, 

 considered by itself, still retains at least a part of its ancestral func- 

 tion of saprophyte : it has acquired in addition the habits of an 

 obligative symbiont upon the enclosed alga. The algal symbiont, 

 whose function is that of assimilating CO , must be looked upon as a 

 facultative symbiont, since it can exist and mature independently. As 

 already stated, the fungal symbiont is incapable of maturing into an 

 independent fungus ; it is absolutely dependent upon the mutualistic 

 association. Considering the lichen as a unit we find that the fungal 

 portion supplies the symbiotic algae with water containing soluble 

 food substances taken from the substratum ; in return the algae 

 supply the fungi with products of assimilation (carbon compounds). 

 The question bearing directly upon the discussion of individ- 

 ualism is whether lichens shall be considered as autonomous struc- 

 tures or not ; that is, shall we consider lichens as modified fungi in 

 agreement with Schwendener and his followers, or shall we consider 

 them as a distinct class in agreement with Reinke ? From the very 

 nature of individualism it is evident that the resulting structure is a 

 morphological unit in the full sense of the word. That is, a lichen 

 is neither fungus nor alga, but a new individual which should be 

 given a definite position in the vegetable kingdom. It is an inde- 

 pendent individual because we find that on separating the symbionts 

 the individual is destroyed, as has already been indicated. There is, 

 perhaps, no doubt that in the lowest forms of lichens the fungal 

 symbiont may develop to maturity without its algal symbiont, but 

 this is not the case in the higher forms. These probable exceptions 

 to the rule do not disprove that lichens are autonomous, nor do they 

 warrant the method of classification adopted by Schwendener. 



