OCCURRENCE OF NITRATE 87 



of shade gave strong positive reactions. Rumex conglomerates 

 growing on an old manure heap gave much stronger reactions 

 for nitrate than another plant of the same species growing a 

 few yards away in a hedgerow. From these and many similar 

 observations, the conclusion is warranted that the presence of 

 detectable free nitrogen only is to be expected when the uptake 

 from the soil is greater than its rate of elaboration, and this 

 relationship in vigorous vegetation is determined by the quality 

 of the soil and the habitat of the plant particularly with regard 

 to illumination. With respect to diurnal and seasonal vari- 

 ation, Anderson observed that in Solatium dulcamara the 

 amount of nitrate in the morning may be considerably less 

 than in the late evening ; and Mercurialis perennis gave posi- 

 tive reactions in October and negative in June. Further, 

 there is a localized distribution in the plant ; thus although 

 nitrate was found in the leaves of various Solanaceae, none was 

 found in the young green berries ; similarly although nitrate 

 occurred in the etiolated inner leaves of a cabbage, none was 

 found in the outer green leaves. 



Nitrate as such is a comparatively inert substance and does 

 not readily lend itself to chemical change, wherefore in the 

 plant it must presumably be changed into a more labile sub- 

 stance before it is elaborated into proteins ; nitrite immediately 

 is suggested on chemical grounds, so that two problems arise : 

 the occurrence of nitrite in the plant and the mechanism 

 which effects the reduction of nitrate to nitrite. 



OCCURRENCE OF NITRITE. 



The literature of the subject shows some disagreement 

 about the presence of nitrite in the green plant. This is due 

 to the following facts. The amount of nitrite is always small, 

 so that its presence must be detected by delicate colour reac- 

 tions ; Griess Ilosvay reagent is commonly used ; it gives 

 a pink colour with dilute solutions of free nitrite, but it is acid 

 in reaction, and thus a pink colour also is given if traces of 

 anthocyanins are present. 



The expressed juice of the plant must be used ; this is a 

 solution of various substances which may react with the 



