388 THE FOOD OF PLANTS 



render possible vital manifestations of which the plant was previously 

 incapable, or suppress others which are no longer necessary. Thus the 

 presence of sufficient sugar suppresses the formation of diastase in many 

 fungi, and it is probably owing to the inhibitory action of the other food- 

 materials that under certain circumstances glycerine and both forms of 

 tartaric acid are not assimilated although they are always absorbed. 



It is by these and similar means that the whole series of processes which 

 constitute metabolism are linked together in a regulatory manner, and in 

 the interactions between cells and organs selective affinity and protective 

 power play a very important part. Thus in time of abundance the stored 

 nutrient materials remain intact, but are translocated and consumed as soon 

 as required, while starvation may cause the re-assimilation of metabolic pro- 

 ducts which under normal conditions remain permanently withdrawn from 

 metabolism. Fungi afford eminently suitable experimental material for the 

 investigation of these fundamental problems, owing to the fact that they can 

 be grown upon a great variety of nutrient media and under widely different 

 conditions, while at the same time the consumption and the resulting 

 growth can readily be determined. 



PART IV 



THE ASSIMILATION OF NITROGEN 



SECTION 68. General View. 



EVERY plant must be supplied with nitrogen in some form or other, 

 for besides the proteid substances which constitute the main bulk of the 

 protoplast, many other organic nitrogen compounds are present as normal 

 plant constituents. Nitrogen, like carbon, is an essential element, and 

 although the latter is more abundant, still more nitrogen is present than 

 of any of the essential elements found in the ash. In the highly nitrogenous 

 seeds of Leguminosae 4-9 per cent, of the dry weight is composed of 

 nitrogen, in the seeds of cereals 2-3 per cent., in bulbs, vegetables, and 

 leaves, from 1-5-6 per cent., so that in a turgid plant from 0-3 to 1-2 

 per cent, by weight of nitrogen may be present l . 



1 Numerous analyses by Konig, Chem. Zus.-setzung. d. menschl. Nahr.- u. Genussmittel, 1889, 

 3. Aufl. Cf. also Sects. 1 1 and 79. For analyses of bacteria and fungi, see Cramer, Archiv f. 

 Hygiene, 1893, Bd. XVI, p. 151 ; Nischimura, ibid., 1893, Bd. xvm, p. 318. According to Fermi 

 (Centralbl. f. Bact., Abth. ii, 1896, Bd. II, p. 512), various micro-organisms, including Saccharo- 

 mycetes and mould fungi, may be grown upon non-nitrogenous media, and then contain no nitrogen, 

 so that this element is apparently non-essential in such cases (cf. Sect. n). The statement is, 

 however, one which requires further proof. 



