CHAPTER VI. 



THE POLYPHYLOGENY OF LICHENS. 



I. INTRODUCTION. 



There is no doubt that the manifold variations observed in the 

 development of lichens are adaptations in favor of the function of as- 

 similation. We have also seen that lichens considered from a purely 

 physiological standpoint resemble the lower chlorophyll-bearing 

 plants (algae), but considered from a superficial morphological stand- 

 point the majority of them resemble the ascomycetous fungi. It is 

 now our purpose to discuss the phyletic relationship of lichens to the 

 fungi and algae. 



Upon careful examination it becomes evident that all forms of 

 lichen can not have originated from a single fungal ancestor entering 

 into a symbiotic association with a single species of alga, that is, 

 it seems evident that their origin is polyphyletic and not monophy- 

 letic. Briefly stated, this means that the various phyletic series or 

 branches took their origin from different fungal ancestors or types in 

 association with certain algal ancestors. To study out the ancestral 

 fungal types is at present impossible for several reasons ; first of 

 all, it is more than likely that the species of fungi which originally 

 entered into the formation of the lichen-types no longer exist in their 

 original form ; that is, they have been converted into other forms or 

 have most likely gone out of existence. It seems quite reasonable 

 to assume that they have gone entirely out of existence as they were 

 apparently dependent upon the symbiotic association for their main- 

 tenance. It must also be remembered that the fungal portion of the 

 lichen has undergone considerable structural as well as functional 

 change during its phylogenetic history. Both of these factors just 

 mentioned make it impossible to determine the exact fungal ances- 

 tors, and this is especially true in the case of the higher lichens. We 

 can, however, in a general way, determine what comprehensive 

 groups of fungi resemble more or less closely the ancestors which 

 entered into the formation of the lichen-types. The exact limitation 

 of a given phyletic group of lichens is very uncertain, since we can 

 not always determine how frequently a species may have changed its 



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