SOURCES OF NITROGEN FOR THE PLANT. 327 



861. There can be found in a large number of plants a small 

 amount of certain matters termed alkaloids, which contain a defi- 

 nite percentage of nitrogen. Such are morphia in the poppy, 

 quinia in Peruvian bark, caffeine in coffee, etc. (see 961). 



862. In most analyses the combined nitrogen of the plant is 

 usually rendered as "albuminoid." The percentages in a few 

 cases are here given : 1 



Red clover, full blossom 3.7 



Sugar beets 8 to 1.0 



Carrot root 1.5 



Carrot leaves 3.2 



Cabbage 1.5 



Winter wheat 13.0 



Beans (field) 25.5 



Apples 22 to .52 



Of these amounts about 16 per cent may be roughly estimated 

 as the content of nitrogen. Therefore in such a case as the 

 carrot root above mentioned, the total amount of nitrogen is 

 really very small (0.24 per cent) ; but the presence of this small 

 percentage is absolutely essential to the life as well as to the 

 health of the plant. 



863. Reserving for a later chapter all consideration of the 

 numerous chemical transformations which nitrogenous matters 

 may undergo in the plant, it is necessary to ask now, (1) whence 

 can the plant obtain adequate supplies of available nitrogen, and 

 (2) how can the plant appropriate, or, to use an equivalent term, 

 assimilate them. 



864. Sources of nitrogen for the plant. It must first be shown 

 whence nitrogen is not supplied. The free nitrogen of the at- 

 mosphere does not appear to be directly available for plants. 

 Although most of the higher plants possess an aerating system 

 (see p. 300), through which atmospheric air can easily enter and 

 traverse the plant and be brought into contact with the tissues, 

 the nitrogen which forms so large a part of the atmosphere is 

 not utilized. This is the interpretation of experiments in cul- 

 ture in which every kind of combined nitrogen is carefully ex- 

 cluded from the plants while they have, at the same time the 

 free nitrogen of the atmosphere in an unlimited supply. 



865. The earliest 2 systematic investigations relative to the 



1 For other cases the student should consult the tables in the Appendix to 

 Johnson's "How Crops Grow," ]868, pp. 385-392. 



2 The following citations refer to earlier observations, none of which, how- 

 ever, can be considered as having fixed any important points relative to the use 

 of atmospheric nitrogen by plants : 



