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lichens were the result of the parasitism of fungi with algae, a 
conclusion without any physiological basis, as later investigators 
have demonstrated. 
Eminent investigators, after years of careful study attempting to 
give a corresponding physiological interpretation to the morpho- 
logical specializations as they occur in the lichen-thallus, have con- 
cluded that it cannot be compared to any fungal structure. 
The recent progress in the study of the phenomena of mutual- 
istic symbiosis has a very important bearing upon the recognition 
of the true nature of lichens. There are botanists who are scien- 
tifically so unprogressive as to recognize no other form of sym- 
biosis * than “ parasitism” (antagonistic symbiosis). Such will, of 
course, persist in maintaining the “ parasitic’? nature of lichens. 
There are all gradations between mutual antagonistic symbiosis 
(mutual parasitism) and complete individualism.; Upon a recog- 
nition of these phenomena depends the proper consideration and 
treatment of lichens. Furthermore, the phenomena of symbiosis 
have an important bearing upon the modern conception of the 
cell, some problems in evolution, and upon the interdependence 
of plants and animals. It is intended to define and discuss these 
phenomena in some future paper. 
Symbiosis has also a direct bearing upon the consideration of 
what constitutes a morphological unit. When a form of symbi- 
osis has reached the stage of complete individualism there can be 
no doubt that the resulting structure constitutes a morphological 
unit in the true sense of the word. The important question at 
this phase of the subject is whether or not the form of symbiosis 
as it occurs in lichens is sufficiently specialized that the resulting 
structure may be recognized as being autonomous. In attempt 
ing to solve this problem it is necessary to consider the following: 
1, Is the lichen-structure morphologically and physiologically 
distinct from the symbionts? 2. Have the symbionts wholly or 
partially lost the power of independent existence? 
* The term is here used in its broader meaning. It includes all forms of contigu- 
ous associations of two or more morphologically distinct organisms accompanied by # 
loss or acquisition of assimilated food-substances, : 
+ Complete individualism is a form of mutualistic symbiosis in which none of the — 
symbionts can exist independently (examples: some of the higher lichens, the cell). 
Semi-individualism is a form of mutualistic symbiosis in which at least ove of the Kine 
biants cannot exist independently (the Collemas and other lichens). . 
