BACTERIAL SYMBIOSIS 261 



compared with the content of the seed) of o-oo8 grm. ; 

 in sand infected with the micro-organism the plant attained 

 a dry weight of 40*574 grm., and showed a nitrogen gain 

 of 1*049 grm. The proof that the bacterium in isolation 

 could fix nitrogen was difficult. The conditions of 

 existence in pure culture and in symbiosis with the plant 

 are, of course, very different. Golding (1905) has shown 

 that the products of the metaboHsm of the bacillus, which 

 accumulate in artificial culture media, have an adverse effect 

 on the fixation of nitrogen ; when these are removed by 

 suitable means, fixation can be demonstrated. 



The mutual benefit of the two organisms is thus quite 

 clear ; the higher plant receives a supply of nitrogenous 

 compounds, the passage of which from the nodule to the 

 rest of the plant has been demonstrated, and the bacterium 

 receives the balance of its food requirements, chiefly carbon 

 compounds. The relations between the two is not so close 

 as in endotrophic mycorhiza, for either organism can exist 

 very well apart from the other — the leguminous plant if it 

 is grown in soils with normal nitrogenous manuring, the 

 bacterium on suitable culture media. Yet normally they 

 are not independent. For it is evident that in nature only 

 in the plant does the bacterium obtain those exact and 

 peculiar conditions which enable it to synthesise nitrogen 

 compounds from free nitrogen ; and only in conjunction 

 with the bacteria are the leguminous plants able to luxuriate 

 in such barren soils as they commonly affect — the rest- 

 harrow and bird's-foot trefoil on sand, the whin on sandy 

 heaths, the petty whin on peaty heaths, and so on. 



Practical Importance. — ^The importance of the symbiosis 

 for plant life in general must be very great. Fixation of 

 nitrogen takes place in ordinary fertile soils by bacteria in 

 the soil itself. But in poor soils such fixation may be much 

 diminished, and the action of denitrifying bacteria may in 

 some cases adversely affect the nitrogen balance. In poor 

 soils leguminous plants frequently thrive, and their decaying 

 roots and nodules enrich the soil to the benefit of other 

 vegetation. Even on good soils an abundance of leguminous 



