36 



doubtedly nutricism, but it differs from that above described in 

 that the host-plant acts as the transfer agent. It is found that in the 

 majority of cultivated Cycas species 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 bear- 

 ing the greater number of tubercles are in no wise injuriously af- 

 fected. Neither has it been proved that the host receives any ma- 

 terial benefit from this association. There is, however, no doubt 

 that the J^ostoc is dependent upon the host for its food supply. For 

 the time being it may be looked upon as nutricism. 



There are numerous other forms of symbiosis which come under 

 the head of nutricism, but sufficient has been given to explain the 

 subject and to prepare the way to a better understanding of mutual- 

 istic symbiosis. 



III. MUTUALISTIC SYMBIOSIS. 



In mutualistic symbiosis host and parasite form a biological union 

 resulting in mutual benefit. On closer examination two forms 

 may be recognized. In one the symbionts (there may be two or 

 more) are not wholly dependent upon each other for their existence; 

 that is, the symbionts are all capable of leading an independent exist- 

 ence. In the second form at least one of the symbionts is absolutely 

 dependent upon the symbiotic association. The former Tubeuf 

 designates as.mutuatism, the latter as individualism. 



I. MUTUALISM. 



This form of symbiosis is of comparatively recent discovery. 

 Frank, more than any other investigator, has given this subject his at- 

 tention, especially its occurrence among the leguminous plants. In 

 the Leguminosae we find the constant occurrence of certain bacteria 

 (rhizobia) within the root-parenchyma causing the development of 

 tubercles. It is, doubtless, a form of mutualism, although Tubeuf 

 considers it a case of nutricism. The bacteria which infect the root- 

 cells and cause the development of the tubercles are dependent upon 

 the host for their food supply. They are even excessively supplied 

 with nourishment, as is indicated by their enormous development. 

 The host in return receives the nitrogenous compounds formed by 

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



