Relations of Higher Plants to the Formation of Nitrates 11 



wheat plants, although able to obtain nitrogen directly from ammonium 

 salts, grew much better in a solution containing nitrates. 



One feature brought out b}^ the numerous experiments with ammonium 

 salts is the difference between plants of various kinds in respect to their 

 ability to absorb ammonium compounds. When fullj^ worked out, the 

 degree to which this aptitude is possessed may serve as a practical guide 

 in the fertilizing of agricultural plants. 



One of the earlj^ beliefs in regard to plant nutrition was that organic 

 matter as such was directly absorbed by higher plants. This opinion was 

 afterwards entirely replaced by the mineral theory propounded by Liebig; 

 and still later the discovery of the nitrifying process almost disposed com- 

 pletely of the belief that organic nitrogen was a food for higher plants. 

 Possibly plants of different species vary in their ability to use nitrogen in 

 the different organic forms. It is also quite likely that the stage of growth 

 of the plant determines, in some measure, the ability of the plant to utilize 

 organic forms of nitrogen. 



The relation that the various nitrogenous compounds of the soil bear 

 to the physiological processes concerned in absorption is an almost un- 

 touched subject. It is possible that changes occur at the root surface 

 through the action of enzymes secreted by the plant. Nitrates have been 

 found in plants, and so also has ammonia. It is generallj^ held that these 

 substances are absorbed directly. 



Nitrate-reducing enzymes have been found in several species of plants, 

 and it seems probable that reduction is the first step in the assimilation 

 of nitrates. Ritter (1912) found that the first process in the utilization 

 of nitrates by certain molds is a reduction of nitrate to nitrite. If amids 

 are an intermediate step in the formation of proteids in the plant, reduc- 

 tion is necessary. 



It may be that the formation of nitrates is merely a process that facili- 

 tates the absorption of nitrogen by reason of the ease with which nitrates 

 pass through the semipermeable membrane of the root cells, and that the 

 most economical use of soil nitrogen would not necessitate the formation 

 of large quantities of nitrates for the growth of many of the higher plants. 



experiments by other investigators 

 Experiments by the writers of this paper and by a number of other 

 investigators indicate that crops differ from one another in respect to the 



