THE REQUIREMENTS OF PLANTS 31 



found that peas assimilate nitrates and ammonium salts equally well, 

 while wheat showed a decided preference for nitrates. 



None of these preferences has been correlated with any other pro- 

 perty of the plants, nor is it easy to explain the fact, on which all 

 experimenters agree, that plants fed on ammonium salts contain a 

 higher percentage of nitrogen than those fed on nitrates (Table VIII.) : 



TABLE VIII. PERCENTAGE OF NITROGEN IN DRY MATTER OF PLANTS. 



The fact indicates that each unit of nitrogen taken up as ammonia 

 is less effective in the growth process than a unit of nitrogen taken as 

 nitrate, and the plants in spite of their high nitrogen content are really 

 suffering from nitrogen starvation. 



Nitrites are also assimilated so long as the solution is not too con- 

 centrated or too acid. 1 



In spite of a considerable amount of work it is not known whether 

 other nitrogen compounds are assimilated by plants. That many other 

 compounds serve as nitrogen nutrients even without the intervention of 

 bacteria seems to be certain (141), but it has never been shown whether 

 assimilation of the compounds as a whole takes place, or whether there 

 is decomposition at the surface of the root. Most of the supposed 

 assimilated compounds are as a matter of fact more or less easily 

 hydrolysable, or otherwise decomposable, with formation of ammonia, 

 and the decomposition will obviously proceed as fast as the ammonia is 

 removed by the plant. The two factors that determine how far a given 

 compound serves as a nitrogen nutrient are: (i) the ease with which it 

 splits off ammonia, (2) the effect on the plant of the other decomposi- 

 tion products : if these happen to be toxic the whole process stops as 

 soon as they have sufficiently accumulated. 



The normal nitrogenous food of plants is, however, a nitrate, and 

 there is a close connection between the amount supplied and the 

 amount of plant growth which is well shown in Hellriegel and Wil- 

 farth's (130) experiments (Table IX.). 



1 See Perciabosco and Rosso, Staz. Speriment. Agrar. ital., 1909, xlii., 5. 



