THE NUTRITION OF THE ROOT 51 



have been looked upon as parasitic organisms. In the last 

 few years, too, successful inoculations have been mjide ; these 

 consisted in introducing the point of a needle into one of the 

 root tubercles and then pricking with the same needle the 

 healthy root of a plant of the same species. Even micro- 

 scopically the spreading of the organism from the point of 

 inoculation has been observed ; at least in the case of the 

 lupin the tubercle-producing parasite is known to form swarm- 

 spores, which pass over into a zoogloea stage, and this organism 

 is now known to science as Ehiedbium leguminoswrum {Bacillus 

 rad'kicola, Beyerinck). 



In consequence of Hellriegel's experiments, it is probable 

 that several kinds of Rhizobium occur among the Leguminosae. 

 The curious circumstance that the plants, when grown in a 

 soil devoid of nitrogen, pass through a starving condition, in 

 which they draw upon the reserves of the most important 

 assimilating organs before they form the root tubercles, is per- 

 haps also capable of explanation. The roots of all plants con- 

 tain nitrates. (The upper portions of plants only contain these 

 substances occasionally, chiefly in the case of annuals.) If we 

 could, therefore, prove that the organisms which produce the 

 tubercles did not develop in the presence of nitrates, the follow- 

 ing would probably be the state of the case. If the soil is 

 well manured, the parasites enter upon a passive stage (zoogloea 

 stage); the plants grow without drawing upon the atmospheric 

 nitrogen. If the soil is poor in nitrogen, the parasites become 

 partially active, and the formation of tubercles commences. 

 Should the soil be entirely devoid of nitrogen, the roots wUl 

 only contain nitrates as long as those contained within the 

 seed last out. The moment the root becomes poor in nitrogen 

 (that is, the time when the plant draws upon its own organs^ 

 it affords the best culture medium for the parasites, and these 

 produce large numbers of tubercles which assimilate the atmos- 

 pheric nitrogen. 



Horticulturists may, therefore, in the case of pot-culture 

 of leguminous plants, dispense with any sort of nitrogenous 

 manuring. 



