390 THE FOOD OF PLANTS 



possible by the photosynthctic production of sugar, and light appears also 

 to exercise a direct favourable influence upon the assimilation of nitrogen 

 compounds especially in the higher plants (Sect. 72), although it does not 

 follow that it directly affords the energy for the synthesis of proteids. 

 Nor is the energy required necessarily obtained by oxygen-respiration, 

 for many anaerobic bacteria can construct proteids from inorganic nitrogen 

 compounds or even from free nitrogen (Sect. 69). 



The exact mode in which the assimilation of nitrogen or nitrogen- 

 compounds takes place is as yet unknown (Sect. 71), but it is certain 

 that the synthesized amides and proteids are usually immediately re- 

 assimilated, and often undergo marked disintegration. Indeed it is 

 probable that in plants, as well as in animals, the decomposition and 

 reconstruction of proteid continue without cessation and are necessary 

 accompaniments of vital activity. Many fungi seem to possess a pro- 

 nounced power of decomposing proteids, amides, &c., and when fed with 

 albumin often produce large quantities of ammonia as the result of their 

 metabolic activity 1 . Certain bacteria actually evolve free nitrogen, and 

 many ferment-organisms may give off large quantities of this gas and 

 of nitrous oxide 2 . The metabolism of such organisms as these involves 

 a continuous loss of nitrogen, whereas in most plants, and especially in 

 Phanerogams, the nitrogen when once assimilated is husbanded with the 

 utmost care, so that almost the whole of it is preserved, however active 

 metabolism and growth may be 3 . This is rendered possible by the fact 

 that the nitrogenous metabolic products, like those containing sulphur and 

 phosphorus, are such as can be drawn into metabolism again and are not 

 aplastic ones intended to remain as permanent constituents of the plant. 

 In spite of all these economic adaptations a certain quantity of nitrogen is 

 lost by the unavoidable leaf-fall, but even when the natural course of 

 metabolism involves a certain loss of nitrogen it is safe to assume that 

 the plant works as economically as the internal and external conditions 

 allow. Moreover in certain cases excreted nitrogenous products are of 

 marked service to the plant in the form of enzymes or of certain 

 odoriferous substances. 



A volatile nitrogenous compound is evolved by Chenopodium vulvaria and 

 by flowers of Crataegus oxyacantha in such abundance that a glass rod dipped 



1 Cf. Wehmer, Bot. Zeitung, 1891, p. 295, and Sect. 102. 



2 No production of free nitrogen occurs in higher plants, for the observations of Boussingault 

 (Ann. d. chim. et d. phys., 1881, v. ser., T. xxn, p. 433) and others are probably due to faulty 

 methods. Cf. Frank, Untcrs. liber d. Emahrung mit Stickstoff, iSSS, p. 25 (Sep.-abdr. a. d. Landw. 

 Jahrb.). Of more recent works, see Aubert, Rev. gen. d. Bot., 1892, T. IV, p. 280, and Schloesing, 

 Compt. rend., 1895, T. cxx, p. 1278. 



3 For examples, see Schroder, Versuchsst, 1868, Bd. x, p. 493 ; Karsten, ibid., 1870, Bd. xm, 

 p. 176; Sachsse, Kciinung v. Pisum, 1872: Detmer, Physiol.-chem. Unters. iiber Keimung, 1875, 

 p. 68 ; Schulzc u. Urich, Landw. Jahrb., 1876, Bd. V, p. 821 (Lupinus) ; Frank, 1. c., iSSS, p. 25. 



