2o6 



SOIL CONDITIONS AND PLANT GROWTH 



rich soils. The parasitism is beneficial to both parties : the 

 plant gains nitrogen and the organism gains carbohydrates. 



In its general outlines the process has been reproduced 

 artificially. Leguminous plants can be fed with nitrogen 

 compounds and made to grow perfectly without the organism. 

 On the other hand, the organism can be grown on artificial 

 media containing carbohydrates,^ made to pass through all its 

 stages from swarmers to bacteroids, and to fix nitrogen.^ The 

 change to bacteroids is conditioned by the presence of carbo- 

 hydrates or of small quantities of various acids, such as are 

 known to occur in the plant (276). The fixation of nitrogen 

 rapidly comes to a stop unless the resulting compound is 

 removed, as in the plant. Golding has attained this end by 

 an ingenious filtering device, and has thus succeeded in fixing 

 considerable quantities of nitrogen. He has also shown that 

 the reaction of the medium during actual fixation is alkaline, 

 but changes to acid when fixation is stopped by the accumula- 

 tion of nitrogen compounds. An actual loss then seems to 

 set in (109). 



The chemistry of the process is unknown ; even the 

 changes in the carbohydrates of the culture medium have not 

 been worked out. Nitrogen fixation is known to take place 

 in the nodule, which thus becomes richer in nitrogen than the 

 rest of the root,^ and its final product is supposed to be a 

 soluble protein which is passed on to the plant. Phosphates, 

 calcium compounds, and carbon compounds, such as sugars, 



^Harrison and Barlow (125) used maltose: other observers have used an 

 infusion of the host plant. Neumann suggests pentosans {213). 



2 See also (13). 



' Stoklasa's analytical results with yellow lupines (Landw. jfahrb., 1895, 

 xxiv., 827) are: — 



Whiting (Illinois Bull., 179, 1915) has discussed this question also. 



