Schneider : the phenomena of symbiosis. 941 



the fungus, which forms a network about the rootlet, is to sup- 

 ply the tree with 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 benefitted, while no evidence could be 

 found to indicate that the fungus is benefitted. The hyphae 

 always remain on the outside of the root, and therefore form 

 ectotrophic mycorhiza. The endotrophic mycorhiza of orchids 

 have not yet been sufficiently studied to determine their exact 

 nature. Tubeuf designates it as nutricism. 



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

 dently nutricism. 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 shown 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 benefitted. 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, nutri. 

 cism. The crayfish Pagurus Prideauxii is constantly associated 

 with one of the actinias (Adamsia palliata). The latter is said 

 to be absolutely dependent upon the former for its food-sup- 

 ply. The crayfish receives only a slight benefit if any. 



Many other forms of nutricism may come to light when the 

 phenomena of symbiosis are more carefully investigated. 



2. Mutualism. — This form of symbiosis has been recently 

 discovered. Reinke and de Bary among botanists and van Bene- 

 den and Klebs among zoologists were among the first writers on 

 the subject. By mutualism is meant a form of symbiosis in 

 which the symbionts mutually benefit each other but are still 

 capable of leading an independent 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 Leguminosae. 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 



