132 LIVING ORGANISMS OF THE SOIL [chap. 



Hellriegel and Wilfarth, who had been growing different 

 kinds of plants in pots containing pure sand supplied 

 with nutrient salts in solution, and particularly with 

 varying quantities of nitrogen, noticed that certain 

 plants which were being nitrogen-starved suddenly 

 recovered their vigour and began to grow as though 

 they were freely supplied with the missing element, 

 nitrogen. Such plants were always of the leguminous 

 family — peas, beans, clovers, lupins, etc. ; when grown 

 in the sand with little or no combined nitrogen, they 

 began by reaching a stage when they had used up all 

 the nitrogen contained in the seed, after which they 

 became stunted or took on rather a light yellowish 

 colour. Some of them, however, would suddenly 

 recover from this debilitated condition, turn green again, 

 and begin to flourish as before. When such plants were 

 shaken out of the sand in which they were growing, 

 their roots were always found to be studded with small 

 warty nodules, which varied in size from that of a pin's 

 head on clover to something as large as a walnut on the 

 larger perennial leguminous plants. The investigators 

 at last made certain that the presence of these nodules 

 upon the roots was accompanied by an increase in the 

 amount of combined nitrogen in the plant, an increase 

 that could only have been obtained by the fixation of 

 some of the atmospheric nitrogea The nodules were 

 found to be colonies of a peculiar form of bacterium, 

 which must have originally come from the soil, because 

 when leguminous plants were grown on sterile sand with 

 suitable precautions, no formation of nodules nor fixation 

 of nitrogen took place until there had been infection of 

 the plant's roots either by the introduction of a trace of 

 soil or of an infusion of a nodule that had grown on 

 another plant. Once, however, inoculation had been 

 effected, nodules began to appear on the root and fixation 



