39 



3- CONTINGENT MUTUALISM. 



Contingent mutualism is a term designating a form of symbiosis 

 which is quite frequently met with among lichens and elsewhere but 

 which has received little attention. Besides the constant symbiotic 

 algae of lichens enclosed by the tissues of the thallus there occur 

 algae which are always found on the outside of the thallus or which, 

 at least, never penetrate the lichen tissue very deeply. Sometimes 

 these algae occur on the upper surface of the thallus, sometimes on 

 the lower surface, sometimes in or among the apothecia. They may 

 be present in one plant, while in another lichen of the same species 

 none may be found. That is, this form of symbiosis is not constant 

 in its occurrence ; hence, it is designated as contingent. It has not 

 been ascertained experimentally whether it is in reality a mutual- 

 istic, indifferent or antagonistic symbiosis. It certainly does not 

 seem to be antagonistic, because in lichens bearing a large number of 

 these accidental algae we do not find the least indication of any 

 harmful influence ; the lichens thus affected seem, indeed, to be un- 

 commonly thrifty, which suggests an additional or secondary mutu- 

 alism. The fact that this association does not result in any notice- 

 able structural adaptation would seem to be evidence that the elective 

 affinity between the symbionts is only slight. That there is some 

 elective affinity is evident from the fact that one and the same species 

 of algae usually occurs on representatives of certain lichen genera. 

 For example, Sirosi-phon -pulvinatus and a species of Nostoc occur on 

 Umbilicaria and Gyrophora ; Pleurococcus vulgar is on Parmelia and 

 Physcia; Pleurococcus -punctiformis on young Cladonia and Baeo- 

 myces. The three algae mentioned are the principal forms which 

 enter into contingent nutritive association with lichens. 



Under this form of symbiosis I do not include the occurrence of 

 algae upon dead or decaying lichen-thalli, which is of frequent oc- 

 currence and is not a case of symbiosis. Nor do I include the un- 

 mistakably mutualistic symbiosis met with in the so-called " pseudo- 

 lichenes," as Lepra viridis and Amphiloma. 



