246 FIELD CROP PRODUCTION 



analyzing the plant, together with the soil, at the end of 

 the experiment. He found that, while in most cases there 

 was no gain in nitrogen, in some there was an increase of 

 as much as 50 per cent. In every case where there was 

 a gain in nitrogen, it was in a pot where a legume had 

 grown. Three years later, 1886, Hellriegel, a German 

 scientist, solved the perplexing problem. By a series of 

 pot tests with legumes, he found that in sterile soil there 

 was no gain in nitrogen, the plants growing for only a 

 short time before they withered and died. On the other 

 hand, in those pots to which a small quantity of water, 

 leached from a soil that had previously grown the legume 

 successfully, was added at the beginning of the experiment, 

 the plants grew vigorously and there was an increase in 

 the nitrogen content above that which was contained in 

 the seed. Upon examination it was found that the plant 

 showing a gain in nitrogen invariably possessed tubercles 

 or nodules on the roots, while none were to be found on 

 those that showed no gain in nitrogen. Hellriegel there- 

 fore advanced the theory that the bacteria in the nodules 

 have a direct relation to the taking up of free nitrogen by 

 the plant, which has since been established by numerous 

 experiments. Hellriegel, however, was not the first to 

 observe the nodules on the roots of legumes, for in 1687 

 Malpighi, an Italian, wrote about them, calling them 

 galls. For many years the nodules were thought to be 

 the result of disease on the roots. Later, however, they 

 were supposed by some to be enlargements of the roots in 

 which reserve plant food was stored. In 1866 a Russian 

 botanist discovered that the nodules were filled with 

 bacteria. Beijerink was the first to isolate the bacteria 

 and grow them in pure culture on artificial media. He 

 named them Bacillus radicicola. 



