404 THE FOOD OF PLANTS 



for the non-peptonizing yeast-plant when it is converted into peptone by 

 the addition of pepsin 1 . Similarly, the difficulty of absorbing peptone 

 tends to make it a bad food-material for Phanerogams, although in early 

 developmental stages proteids may form their best possible food-material, 

 as, for example, when the reserve materials of the seed are consumed 

 during germination. It must not, however, be forgotten that growth and 

 development may be influenced in a variety of ways by the external 

 conditions. Thus peptone is necessary for a few facultative anaerobes only 

 when the supply of oxygen is deficient, while the nutritive value of 

 a nitrogen-compound may vary in the presence of different carbon-com- 

 pounds 2 . 



Probably when several nitrogen-compounds are supplied they may 

 support one another, or the one may protect the other in a manner similar 

 to that observed in the case of carbon-compounds (Sects. 66, 67). Thus 

 the presence of an abundance of nitrates lessens the amount of free nitrogen 

 assimilated by leguminous plants, but on the other hand ammonia and 

 nitrates do not protect one another in the case of Penicillium glaucum 

 and Aspergillus niger, nor is ammonia protected by the presence of proteids. 

 Similarly in phanerogams ammonium salts are always assimilated when 

 present, however abundant nitrates may be. 



Under normal conditions plants obtain almost the whole of their 

 nitrogen from the soil or surrounding water, while if protected from rain 

 they may still obtain a very trifling amount of nitrogen from volatile 

 nitrogen-compounds, such as ammonia 3 , &c., present in the air. The 

 amount thus obtained may be comparatively large in plants which normally 

 grow upon or near to manure or decaying organic matter from which 

 volatile nitrogen compounds are evolved. 



Phanerogams. Boussingault first showed that nitrates afford the most suitable 

 source of nitrogen for most Phanerogams including Leguminosae, and this result 

 has been frequently confirmed by means of the water-culture method 4 . Accord- 

 ing to Kellner 5 , swamp-rice, especially when young, exhibits a preference for 

 ammonium salts, and this is probably often the case in plants which grow in swampy 

 or badly aereated soil, where the process of nitrification takes place slowly or 



1 Nageli, Bot. Mitth., 1891, Bd. in, p. 419. 



2 For yeast, cf. Chudiakow, Landw. Jahrb., 1894, Bd. xxm, p. 460. 



3 Sachs, Jahresb. d. Agr.-Chem., 1860-1, p. 78; Ad. Mayer, Versuchsst., 1874, Bd. xvn, 

 p. 329; Schlosing, Compt. rend., 1874, T. LXXIV, p. 700; also Altvater, Landw. Jahrb., 1885, 

 Bd. xiv, p. 621. 



4 Boussingault, Agron., &c., 1860, T. I, p. 154; also Ann. d. sci. nat., 1855, iv. ser., T. iv, 

 p. 32, and 1857, iv. ser., T. VI, p. i; Rautenberg und Kuhn, Versuchsst., 1864, Bd. vi, p. 355; 

 Lucanus, ibid., 1865, Bd. VII, p. 364; Hampe, Hosaeus, Birner, Lucanus, and recently Pitsch, Ver- 

 suchsst., 1895, Bd. XLVI, p. 359. 



5 Versuchsst., 1884, Bd. XXX, p. 18. 



