THE NUTRITION OF THE ROOT 49 



are added to it, leguminous plants will be able to grow, but 

 will not produce tubercles. 



The most important fact, however, is that leguminous plants 

 exhibit a very healthy development and yield a good crop in a 

 soil which contains much too little nitrogen for such a crop, 

 or even when the soil is entirely devoid of nitrogen. In plants 

 grown under these conditions the production of root tubercles 

 is very great. It follows from this that these leguminous 

 plants must have absorbed the nitrogen necessary for their 

 development from the air, and that this power of absorption 

 of free nitrogen has some connection with the presence of the 

 root tubercles. We know, therefore, of some plants which 

 are able to make use of the air as an inexhaustible source of 

 nitrogen, and can absorb from it sufficient free uncombined 

 nitrogen to provide for an entire and rich crop. 



This power of making use of the atmospheric nitrogen is, as 

 far as experiments have so far shown, not possessed by any 

 other cultivated plants ; they do not seem to derive any 

 appreciable advantage from it, and for their normal develop- 

 ment the presence of a large amount of nitrogen in the soil is 

 essential. The richness of their crop is approximately pro- 

 portionate to the amount of nitrates in the soil. But there 

 is in reality no fundamental difference in the methods of 

 nutrition between leguminous and other plants ; for the former 

 are able to nourish themselves entirely from nitrates if they 

 are plentifully present in the soil, but they possess in the 

 root tubercles a special provision by which they can supply 

 themselves with nitrogenous food when other plants would 

 not be able to do so. 



This view is the outcome of extensive and conscientious 

 experiments of Hellriegel, upon which experiments we base 

 the subsequent remarks. 



With regard to the faculty of the Leguminosa' to live upon 

 the atmospheric nitrogen, it has been proved that the plants 

 showed a complete and normal development if they were 

 grown uncovered in the open in a soil with little or no 

 nitrogen compounds, but in which the other food substances 

 were present. Their development was furthered, too, in a soil 

 devoid of nitrogen by the addition to the soil of an extract in 



D 



