940 MINNESOTA BOTANICAL STUDIES. 



III. Mutualistic Symbiosis. 



This form of symbiosis differs from the preceeding in that 

 the relationship of the organisms is mutually beneficial. Each 

 symbiont possesses or has developed a specific character which 

 is useful for the other symbionts. As in the preceding 

 forms of symbioses widely different organisms may enter into 

 its formation. The morphological changes accompanying the 

 functional relationships may be very marked or scarcely 

 perceptible, nor is the adaptation quantitatively and qualita- 

 tively equal for all the symbionts. The adaptation is rather 

 complementary, one organism supplies a deficiency (morpho- 

 logical or physiological) of the others. Theoretically there is 

 no limit to the degree of specialization and perfection that 

 this form of symbiosis may attain. In fact mutualistic sym- 

 biosis implies that there is a higher specialization and 

 greater fitness to enter into the struggle for existence. This 

 is most beautifully illustrated in the case of lichens. These 

 plants are of wider distribution and possess greater vitality 

 and physiological activity than either of the symbionts. They 

 occur in the tropics as well as in the extreme north; in the 

 lowest valleys as well as on the highest mountain peaks. 

 Bonnier has shown that their vitality is greater than that of 

 any other plants. Likewise the mutualistic symbiosis occur- 

 ring in the Leguminosae adapts these almost equally well to 

 rich and poor soil thus giving them a great advantage over 

 other plants. Our knowledge of the higher forms of mutual- 

 istic symbiosis is as yet too problematic to permit us to make 

 any authentic statements as to the benefits derived therefrom. 



1. Nutricism. — Nutricism establishes a connecting link be- 

 tween the lesser marked symbioses and mutualism. It may be 

 defined as a form of symbiosis in which one symbiont nour- 

 ishes the second symbiont without receiving any benefit in re- 

 turn. It might therefore be designated as one sided or incom- 

 plete mutualism. Absolute nutricism, as above defined, does 

 perhaps not occur, for, as already indicated, it is not reason- 

 able to assume that any symbiotic relationship exists in which 

 all of the symbionts are not more or less mutually affected. 

 There are, however, a few instances in which one symbiont is 

 very materially benefitted, while the other is not materially 

 benefitted. The most marked example is met with in the 

 mycorhiza of the Cupuliferae. A mycorhiza is the association 

 of a hyphal fungus with the younger rootlets. The function of 



