FIXATION OF NITROGEN 51 



growth of non-leguminous plants was directly pro- 

 portional to the amount of nitrate that they supplied. 

 In the case of leguminous plants there was no rela- 

 tion between the supply of nitrate and the growth 

 of the plant. If nitrate was not supplied to the 

 leguminous plant after the seedling stages had been 

 passed, the growth of the plant was arrested for a 

 short time, and then it either died or started to grow 

 again and did well. Further, on analyzing the soil, 

 they discovered that whereas, as might have been 

 expected, there was always rather less nitrate in the 

 pots in which non-leguminous plants had been 

 grown than at the start of the experiment, in the 

 case of the pots containing the leguminous plants 

 the soil had been considerably enriched with nitrogen. 

 In three cases, for instance, the gain amounted to 

 0*910, 1*242, and 0*789 grammes per pot respectively. 



Botanists had already noted that leguminous 

 plants were peculiar in the fact that their roots con- 

 tained swellings or nodules, and without appreciating 

 the bearing of the discovery, had remarked that 

 these swellings or nodules contained bacteria. 

 Hellriegel and Wilfarth, however, found in the 

 experiments they made in growing leguminous plants 

 in sterilized sand that all the plants that lived con- 

 tained these remarkable swellings on their roots, and 

 they reached at once the right conclusion that legu- 

 minous plants had apparently the power of obtaining 

 nitrogenous food material from the air because of the 

 bacteria harboured in their roots. 



At last the great puzzle had been solved, and it 

 was found that the difference that seemed to mark 



