2o8 THE LIVING ORGANISMS OF THE SOIL [chap. 



yet it becomes so modified by growing in symbiosis 

 with the different leguminous plants, that the best 

 results are only obtained when each species is directly 

 infected from nodules taken from the same kind of plant. 

 Accordingly, he proceeded to the introduction, on a 

 commercial scale, of pure cultivations on a gelatine 

 medium of the races of bacteria appropriate to each of 

 the leguminous plants grown as field crops. The jelly, 

 which was called " Nitragin," was to be dissolved in a 

 large bulk of water and sprinkled over the seed before 

 sowing; thus ensuring inoculation with the appropriate 

 organism, which might not happen to be present in the 

 soil. Nitragin failed to fulfil the expectations which 

 were formed at its introduction, partly because of the 

 nitrogenous character of the medium, in consequence of 

 which the organisms possessed very little vitality or 

 power of fixing nitrogen. Since that time, however, 

 several other methods of cultivating the organism for 

 inoculation purposes have been introduced, either by 

 growing it on an agar jelly, which contains practically 

 no nitrogen (Hiltner), by drying up cotton wool which 

 has been soaked in an active liquid culture (Moore), 

 or by drying soil which has been treated in the same 

 way. Active liquid cultures can now be obtained from 

 most laboratories. The culture thus obtained is added 

 to a large bulk of water, containing a little separated 

 milk to protect the organisms from substances excreted 

 during germination, and the seed is dipped into it and 

 allowed to dry before sowing. The culture may also 

 be sprayed over the ground or absorbed by a large 

 quantity of earth which is afterwards sown. The results 

 of such inoculation are very conflicting; where the land 

 has been regularly under cultivation and has carried 

 the leguminous crop in question many times previously, 

 nodules are practically always formed whether the seed 



