458 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



are for the most part aplastic products and have frequently only a biological 

 importance. 



It probably never happens that the whole of the nitrogen is present 

 in the form of proteid compounds, which contain on the average from 

 50 to 90 per cent, of the nitrogen in vegetative parts. It seems moreover 

 that the non-proteid nitrogenous compounds diminish in amount only when 

 a deficient supply of nitrogen causes the plant to economize its resources as 

 far as possible. Apparently these nitrogenous compounds include both 

 plastic and aplastic material. Soluble amides and amido-acids appear as 

 reserve-materials chiefly in rhizomes, roots, bulbs, and tubers, organs which 

 arc not normally subject to desiccation, and in such cases almost the whole 

 of the nitrogen may be present in this form. On the other hand in seeds, 

 and apparently in spores also, proteids predominate, so that usually only 

 from 2 to 10 per cent, of the total amount of nitrogen is represented by 

 non-proteid compounds '. During germination the proteids decompose into 

 amides, &c., and these may be formed in such abundance when leguminous 

 seedlings are grown in darkness that 50 to 75 per cent, of their nitrogen 

 may be present in the form of non-proteid substances. 



Schulze and his pupils were the first to recognize that various amides were 

 common products of plant-metabolism, but the formation and accumulation of 

 asparagin was known before their researches 2 . By isolating the different amides, 

 approximate quantitative determinations have been made of the total amount of 

 nitrogen present, and an estimate of the amount of nitrogen held in the form 

 of amides may be directly obtained by treating triturated plants with nitrous acid 

 and hydrochloric acid, estimating the nitrogen in the form of ammonia 3 . This 

 method is by no means an exact one, nor is it possible to determine with precise 

 accuracy the amount of nitrogen present in the form of proteid. 



The amount of these metabolic products varies in different stages of develop- 

 ment : thus during germination the relative amount of amides increases until 

 10 to 30 per cent, of the nitrogen may be represented by such substances, or in 

 Litpinus, Vicia, and other Leguminosae as much as 75 per cent., mainly in the 

 form of asparagin, although traces of glutamin, leucin,&c. are present. In seedlings 

 of Cncurbita the total amount of amides present is small and glutamin pre- 

 dominates, whereas in seedlings of Abies pectinata and other Coniferae arginin is 

 the most abundant amide 4 . The relative amounts present are, however, in all 

 cases liable to pronounced fluctuations. 



1 E. Schulze, Landw. Jahrb., iSSS, Bd. XVII, p. 693. 



2 Cf. the literature given by Pfeffer, Jahrb. f. wiss. Bot., 1872, Bd. VIII, p. 5^7. Most of 

 Schulzc's works, &c. since 1876 are published in Landw. Jahrb., in Versuchsst., and in Zeitschr. f. 

 physiol. Chemie. 



3 Cf. Kb'nig, Unters. lamhv. wichtiger Stoffe, 1891, p. 214; E. Schulze, Versuchsst., 1883, 

 Bd. XXIX, p. 400; 1884, Bd. XXX. p. 459; 1887, Bd. xxxili, p. 124. To calculate the amount of 

 proteids present by multiplying the amount of nitrogen found by 6-25 is obviously inaccurate. 



* Cf. tables, Sect. 81. Schulze (I.e., In wie weit, &c.) gives a summary of the literature, and 



