Petkie. — On Vegetable Physiology. 431 



starved condition supervened. In these experiments it was 

 found that, within the hmits of the optimum supply, there 

 was a direct proportion between the amount of nitrates added 

 and the yield of grain. 



If, instead of nitrates, ammonia salts or other nitrogenous 

 compounds were added the resumption of growth was delayed ; 

 and only after a pause of considerable length did these salts 

 become available as nutriment. In this case the iiicrease in 

 the yield of grain was not proportional to the amount of 

 ammonia added. 



These results threw considerable light on a question that 

 had been much debated. Justus von Liebig and others taught 

 that ammonia and salts of ammonia were the most desirable 

 form in which plants could receive their supplies of nitrogen. 

 Hellriegel's results make this view extremely doubtful. The 

 evidence irresistibly suggests that the salts of ammonia had to 

 undergo nitrification — i.e., had to be oxidized into nitrates — 

 before the roots of the grasses could avail themselves of their 

 stores of combined nitrogen. As this has little direct bearing 

 on my immediate subject, I need only say that Hellriegel con- 

 siders that nitric acid and its salts are the only directly avail- 

 able sources of nitrogen for gramineous plants. The main 

 result of this series of experiments was the proof that the 

 Graminege are entirely dependent on the combined nitrogen in 

 the soil for their supplies of nitrogen. They cannot draw upon 

 the stores of free nitrogen in the air, except in so far as rain or 

 dew carry down nitrogenous compounds into the soil. 



Hellriegel then proceeds to show that similar experiments 

 with leguminous plants yield totally different results. In a 

 soil devoid of nitrogen and protected from rain, exactly like 

 that in which the gramineous plants had uniformly starved, 

 peas were allowed to germinate and grow ; and in nearly every 

 case they flourished and yielded a large increase. Thus peas 

 grown in small culture-vessels little larger than a thimble 

 yielded above ground between two and three times the amount 

 of dry substance that the seeds contained, and this, of course, 

 without any addition of nitrates to the soil in which they 

 grew. The plants grew normally throughout, and even vigor- 

 ously. Hellriegel notes that such a yield of dry substance 

 from the same kind and quantity of soil could not have been 

 obtained with a gramineous plant such as barley, even with 

 the addition of a sufficiency of nitrates. The result which 

 Lawes, Gilbert, and Pugh had reached by analysis of legumi- 

 nous plants — namely, that these plants absorb and utilise much 

 more nitrogen than the soil can supply — was placed once for 

 all on a firm basis of trustworthy experiment. 



The source of the nitrogen gained by these pea-plants had 

 now to be considered. The soil contained none, for it con- 



