50 THE PHYSIOLOGY OF PLANTS 



distilled water to the extent of 1—2 per cent, of a soil in which 

 the same leguminous plants- had formerly been grown. After 

 such an addition, the development of the plants was not only 

 normal, but often indeed luxurious ; but their mode of develop- 

 ment was very different from plants of the same species and 

 origin which absorbed their nitrogen in the. form of nitrates 

 from a sterilised soil. For while the latter exhibited a con- 

 tinuous and even growth from the moment of germination, 

 the plants grown in a soil devoid of nitrates passed through 

 a very marked and characteristic phase, exhibiting the signs 

 of starvation, and this was followed, after a longer or shorter 

 period, by a very active growth. 



Such a beneficial extract cannot, however, be made from 

 every soil, and even an active extract becomes powerless if it 

 is boiled, or only heated up to 70° C. There is a diflFerence, too, 

 in the way in which leguminous plants of the same species 

 react to extracts from different soils. Thus an extract of a 

 soil in which peas and clover had been regularly grown for a 

 considerable period, but on which lupins and Seradella had 

 never yet been grown, was only beneficial for the growth of 

 peas in the experimental soil, but was perfectly useless for the 

 growth of lupins or Seradella. Lastly, the very interesting 

 experiment was tried of growing a leguminous plant in such 

 a way that one half of the root grew in a solution devoid of 

 nitrogen, but supplied with such a soil extract, while the other 

 half of the root grew in a nutritive solution of the same 

 composition to which the soil extract had been added after 

 it had been sterilised by boiling. The result was that root 

 tubercles were only developed in the solution which had not 

 been sterilised. 



All these results point to the conclusion that the develop- 

 ment of root tubercles — that is, the formation of centres of 

 assimilation of atmospheric nitrogen — depends on the develop- 

 ment of organisms within the roots of leguminous plants. 

 These organisms are probably different for the different kinds of 

 Leguminosse, and multiply in the soil in which these leguminous 

 plants have once been grown. This supposition is strengthened 

 by the fact that for years various observers have described 

 within the substance of the root tubercles small bodies which 



