Petrie. — On Vegetable Physiology. 433 



before entering Nos. 2, 3, and 4 the stream of air was deprived 

 of any ammonia or nitric acid it contained. The plants all 

 grew normally, and passed successfully through the usual 

 starvation stage. When the experiment was stopped they 

 had grown to a height of 120cm., and had entered on the 

 flowering and fruiting stage. Subsequent analysis showed 

 that the plant grown in the ordinary air did not yield as much 

 dry substance in the straw and roots together as did the 

 plants grown in the purified air. Several repetitions of this 

 experiment made it quite clear that the small traces of com- 

 bined nitrogen present in the air did not supply the experi- 

 mental plants with the nitrogen which they somehow 

 obtained. The only hypothesis that could now be enter- 

 tained was that somewhere and somehow the leguminous 

 plants do make the free nitrogen of the air available for their 

 nutrition. 



In the course of Hellriegel's experiments he noticed that, 

 while some plants grew in a soil devoid of nitrogen and under 

 the conditions already described, others grew badly, and some 

 never recovered from the starvation stage. Eepeated and 

 careful examination of the experimental plants brought to 

 light a remarkable fact. The plants that remained in the 

 starvation phase had either no tubercles on their roots or 

 very few and insignificant ones, whereas the plants that 

 throve and grew luxuriantly had many well-developed tuber- 

 cles on their roots. The more plants Hellriegel investigated 

 the more was he convinced that the development of the root- 

 tubercles stood in the closest and most direct relation to the 

 growth and assimilation of the whole plant. The origin of 

 the root-tubercles had now to be investigated. It was likely 

 that they were caused by the invasion of the root-tissues at 

 certain spots by micro-organisms existing in the soil. To test 

 this supposition Hellriegel carried out numerous experiments. 

 A brief account of two of these will show his methods and his 

 results. 



On a certain day (25th May) were taken forty vessels 

 filled with soil devoid of nitrogen, and two pea-seeds were 

 planted in each. Ten of the vessels were then watered with 

 washings from the fertile soil of the culture field in which 

 micro - organisms existed in abundance. Thereafter these 

 vessels, and from the first all the others, were watered with 

 pure distilled water. In about three weeks the aspect of the 

 plants was changing, and all turned pale as the seed reserves 

 were exhausted. So far there was no difference to be seen 

 between the forty cultures. But a difterence soon set in, and 

 in another week it was most decided. In the ten vessels 

 supplied with bacteroids all the plants had regained their 

 fresh-green colour, and commenced to grow vigorously. Of 

 28 



