NUTRITIVE VALUE OF DIFFERENT NITROGEN COMPOUNDS 403 



Hellriegel and Boussingault have shown, a small quantity of volatile nitrogen com- 

 pounds may have been absorbed from the air. 



Schlosing and Laurent proved in 1892 that leguminous plants absorb nitrogen 

 from the air, whereas the amount does not diminish in vessels enclosing other 

 plants. Frank ' supposed that all plants were able both to evolve and to fix 

 nitrogen, both of which properties seem however to be possessed by a few special 

 forms only. Nitrogen is apparently evolved only by certain bacteria (Sect. 68), 

 and higher plants do not exhale any nitrogen after they have been kept for some 

 time in an atmosphere composed of oxygen and hydrogen. 



SECTION 70. The Nutritive Value of Different Nitrogen 



Compounds. 



Most phanerogamic plants grow best when supplied with nitrates, 

 but ammonium salts are more suitable for others, and many hetero- 

 trophic organisms either require a supply of peptone or other proteids, 

 or attain their maximum development only when supplied with nitrogen 

 in the form of proteids or amides. Hence it is possible to distinguish, in 

 addition to the plants which assimilate free nitrogen, those which require 

 (i) nitrates or ammonium salts, (2) amides, (3) peptones or other proteids 2 . 

 Many plants may obtain suitable food by assimilating proteids or amides, 

 whereas others must be supplied with carbon compounds as well (Sect. 66). 

 No sharp line of demarcation can be drawn between any of these classes of 

 plants, for although many are comparatively restricted in their require- 

 ments, others may be satisfied by a great variety of nitrogen-compounds, 

 and may simply exhibit a more or less marked preference for proteids or 

 nitrates as the case may be. It is easy to understand why proteids may 

 not form a good source of nitrogen for those plants which normally 

 construct their proteids from inorganic nitrogen-compounds, for the disuse 

 of any vital activity is apt ultimately to exercise certain injurious effects : 

 thus Clostridium Pasteurianum is unable to develop when the absence of 

 free nitrogen prevents the exercise of its normal assimilatory activity. 



The causes of the high or low nutritive value of a given nitrogen- 

 compound are not indicated by the effects which it produces upon growth 

 and development. The nutritive value of a substance is mainly deter- 

 mined by the specific nature of the plant examined, although other 

 factors may also come into play, such as solubility and absorptive power. 

 Solid proteids are available only for those plants which excrete peptic 

 or tryptic ferments, so that albumin only forms a good source of nitrogen 



1 Frank, Ber. d. Bot. Ges., 1886, pp. 293, 380. 

 - Beyerinck, Bot. Zeitung, 890, p. 731, footnote. 



D d 2 



