37 



is true that the benefit received by the host is far greater than that 

 received by the infecting bacteria, but it is nevertheless mutual. It 

 has been demonstrated experimentally that the host may thrive with- 

 out the symbiotic bacteria and that the bacteria occur normally in the 

 soil and that they will readily develop in artificial culture media. 

 This shows conclusively that the symbionts are not absolutely de- 

 pendant upon each other. Host and parasite, however, thrive much 

 better when in symbiotic association, especially in poor soil. 



2. INDIVIDUALISM. 



In individualism is reached the acme of mutualistic association. 

 As already explained, the conditions of this occurrence require 

 that one of the symbionts be absolutely dependent upon the mutual 

 relationship. The phenomenon is typically met with in lichens. In 

 these plants we find the nutritive interdependence so marked that a 

 new individual is formed, which in its morphology and physiology is 

 wholly different from any of the symbionts. From the historical re- 

 view of the Schwendenerian period we learned that in lichens we 

 have an association of a fungus with an alga or algae. It has been 

 demonstrated experimentally that while the algal portion is capable 

 of existing independently, the fungal portion can not do so, that is, 

 the symbionts are not mutually dependent. Individualism will have 

 reached its highest development when all the members of the symbi- 

 otic relationship are incapable of existing independently. There is 

 no doubt that in time this will be the case in our present lichens. 

 Since the fungus has lost its power of independent existence as a re- 

 sult of the symbiotic association it is reasonable to assume that the 

 alga likewise is undergoing a change indicative of a more highly 

 developed dependence upon this association. It seems quite evident 

 that it is more difficult to cultivate the lichen-algae than the corre- 

 sponding free forms. 



We shall now very briefly explain the difference between individ- 



! The student is advised to consult the following works giving a condensed review 

 of the phenomenon of mutualism as it occurs in the Leguminosae : 



Frank, A. B. Lehrbuch der Botanik, i : pp. 255-275. Leipzig. 1892. 



Atkinson, G. F. Contribution to the biology of the organisms causing leguminous 

 tubercles. Bot. Gaz. 18 : 157, 226, 257. 1893. 



Schneider, A. Observations on some American Rhizobia. Bull. Torr. Bot. Club, 

 19 : 205-218. 1892. 



