SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 151 



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

 gus, which forms a network about the tips of the terminal rootlet, is to 

 supply the tree with certain food-substances and moisture taken from the 

 soil. It also supplants the function of the hair-cells which are wanting in 

 the mycorhiza. It has been proved, experimentally, that the tree is greatly 

 benefited, while no evidence could be found to indicate that the fungus is 

 benefited. The hyphae always remain on the outside of the root, and 

 therefore form an ectotrophic association. The endotrophic mycorrhiza 

 of orchids have not yet been sufficiently studied to determine the 

 kind .of symbiosis which they represent. Tubeuf designates it as 

 nutricism. 



In Cycas revoluta we find a form of symbiosis which is evidently nutri- 

 cism. It is found that in the majority of cultivated cycads there are 

 numerous tubercular outgrowths from the roots, which usually contain a 

 species of Nostoc between the cells of a specialized parenchyma. This is 

 evidently not a form of parasitism as is indicated by the fact that the 

 cycads bearing the greater number of tubercles are in' no wise injuriously 

 affected; neither has it been proven that the host is benefited. There is, 

 however, no doubt that the Nostoc is dependent upon the host for its food 

 supply. It may therefore be looked upon 'as a case of nutricism, in which 

 the host acts as the transfer agent. 



Klebs cites an interesting example which is, no doubt, nutricism. The 

 crayfish Pagurus Prideauxii is quite uniformly infested by one of the 

 actinias (Adamsia palliata). The latter is said to be absolutely dependent 

 upon the former for its food-supply. The crayfish receives only a slight 

 benefit if any. 



2. Mutualism. This form of symbiosis was first described byReinke 

 and de Bary among botanists and van Beneden and Klebs among zoolo- 

 gists. By mutualism is meant a form of symbiosis in which the symbionts 

 mutually benefit each other, but are still capable of leading an independ- 

 ent existence. It is an association of wide occurrence and in many 

 instances reaches a high degree of morphological and physiological 

 specialization. 



The most striking example occurs in the root-tubercles of the Legum- 

 inosae. The tubercles are neoformations induced by the Rhizobia which 

 grow and multiply in the parenchyma cells. The Rhizobia take their food 

 supply direct from the plasmic and other cell contents of the host; in 

 return the latter receives certain nitrogenous compounds formed by the 

 bacteria in the process of binding the free nitrogen of the air. It has been 

 proven experimentally that the symbionts may exist independently, but 

 thrive much better when in association, especially in poor soil. 



To this category also belong the association of ants and trees in the 



