SYMBIOSIS 



133 



or between bacteria and higher plants. The partnerships between 

 hermit crabs and sea-anemones and the like are sometimes defined 

 by the term commensalism (joint diet), which is applied to such 

 associations having negative results, neither partner gaining much 

 advantage from the association. Symbiosis and commensalism must 

 be distinguished from parasitism, which indicates that all the 

 advantage is on the side of the parasite, and nothing but loss on 

 the side of the host. Association 

 of organisms together for increase 

 of virulence and function should 

 be distinguished from symbiosis, 

 and mere existence of two or more 

 species of bacteria in one medium 

 is not, of course, symbiosis. Most 

 frequently such a condition would 

 result in injury and the subsequent 

 death of the weaker partner, an 

 effect precisely opposite to that de- 

 fined by this term. 



The example of bacteriological 

 symbiosis with which we are con- 

 cerned here is that partnership be- 

 tween bacteria and some of the 

 higher plants (Leguminosse) for the 

 purpose of fixing nitrogen in the 

 plant and in the surrounding soil.* 



The nitrogen-fixing bacteria, 

 the third group of micro-organisms 

 connected with the soil, exist in 

 groups and colonies situated inside 

 the nodules, appearing, under cer- 

 tain circumstances, on the rootlets 

 of the pea, beau, and other Legu- 

 minosse. It was Hellriegel and 

 Wilfarth who first pointed out 

 that, although the higher chloro- 

 phyllous plants could not directly 

 obtain or utilise free nitrogen, 

 some of them at any rate could acquire nitrogen brought into com- 

 bination under the influence of bacteria. Hellriegel found that the 

 gramineous, polygonaceous, cruciferous, and other orders depended 



Fig. 19 Rootlet of Pea with Nodules. 



* Examples of bacteria symbionts are numerous ; e.g. the dissolution of cellulose 

 (Van Senus) ; the decomposition of sound potato in water exhausted of air (Ward) ; 

 the reduction of sulphates ; the oxidation of sulphuretted hydrogen ; the iron 

 bacteria, etc. 



