NITROGENOUS METABOLISM 461 



proteid not digestible by pepsin decreases slightly at first, although the number 

 of cells and protoplasts increases. The amount increases again as cellular 

 multiplication continues, and in many cases this becomes evident at a comparatively 

 early stage of development (Triticitm vulgare, Helianthus}\ 



The nutritive value for mankind decreases with an increase of the percentage 

 of proteids difficult of digestion, and also of asparagin, whereas the latter affords 

 a very good source of nitrogen for many plants. 



In turgid plants the reserve-proteids are for the most part present in dissolved 

 form, but in certain cases they occur as solid crystalloids 2 . 



In seeds the proteids occur in the form of aleurone grains 3 with or without 

 crystalloids, and these may be partially soluble in saline solutions, or even in 

 pure water if the latter dissolves some of the salts associated with them. (Pfeffer, 

 I.e., 1872, p. 491). 



SECTION 80. Nitrogenous Metabolism. 



<i 



One function of metabolism is to render the absorbed food-materials of 

 service to the plant, and the changes involved may be either constructive or 

 destructive. In the latter case they sometimes lead to the partial or complete 

 disorganization of proteid molecules, so that amides, amido-acids, ammonia, 

 or in a few cases even nitrogen may be produced, especially when the plant 

 stands in need of such substances and when they are absent from the food 

 supplied or present in insufficient amount. This takes place when seeds of 

 Liipinus, Vicia, &c. germinate, for almost the whole of the nitrogen of the 

 seed is present in the form of proteids, whereas in the seedling as much as 

 30 per cent, of the dry weight may consist of asparagin 4 . There can be 

 no doubt that amides may also be formed synthetically, and hence it is 

 not always possible to tell the precise mode of origin of the different 

 nitrogenous compounds which may appear (Sects. 68, 71). 



In a fungus fed solely with proteids and inorganic salts not only must 

 the entire supply of energy be obtained from the partial or complete com- 

 bustion or disintegration of proteids, but the cell-wall, carbohydrates, fats, 

 and indeed every metabolic product must directly or indirectly be derived 

 from the same source. When this is the case ammonia is a constant end- 



1 Palladia, Rev. gen. d. Bot, 1896, T. vnr, p. 228 ; also Frankfurt, Versuchsst, 1893, Bd. XLIII. 

 p. 175 ; Prianischnikow, ibid., 1894, Bd. XLV, p. 253. 



2 These are probably vitellinates of alkaline earths. Schmiedeberg, Zeitschr. f. physiol. Chemie, 

 1877, Bd. I, p. 205 ; Griibler, Uber krystallinisches Eiweiss d. Kiirbissamen, 1881. 



3 On aleurone grains, see Pfeffer, Jahrb. f. wiss. Bot.. 1872, Bd. vin, p. 429: Wakker, ibid., 

 1888, Bd. XIX, p. 453; Liidtke, ibid., 1889, Bd. XXI, p. 62. All observers agree that the aleurone 

 grains are formed in the cell-sap, but the grains possibly do not always differentiate in the same 

 manner. Cf. Pfeffer, Aufnahme u. Ausgabe ungeloster Korper. 1890, p. iSo. On Globoids, cf. 

 Sect. 74. 



1 Cf. E. Schulze, Landw. Jnhrb.. iS^8. Bd. XX t. p. 694; iSSo. Bd. IX. p. 12. 



