460 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



in a living cell will not be more than 8 per cent, of the total dry weight, or 

 2 per cent, of the total weight when fresh. This corresponds with the amount 

 of nitrogen present (Sect. 68), although a portion of the latter is always 

 present in the form of non-proteid substances. In storage-receptacles, such 

 as the seeds of Leguminosae, proteids may form 20 to 35 per cent, of the 

 dry weight, and in the grains of cereals about n per cent. 1 Protoplasm 

 contains many nucleins, some rich in phosphorus and others poor in this 

 element. Globulins and vitellins, as well as albumoses 2 , form the commoner 

 forms of rescrve-proteid, and traces of peptone may also be found (Sect. 91). 

 Nuclco-protcids and mucin often accumulate in large amount and are 

 no doubt used along with many true proteids as plastic material. Indeed 

 it would be surprising were not this function especially prominent among 

 proteids in general. 



It is not at present possible either to determine or to localize all the different 

 forms of proteid present in the cell, but in general a distinction may be made 

 between constructive and plastic proteids. The skeletal framework of the protoplasm 

 seems to be mainly composed of proteids which are comparatively resistant to 

 peptic digestion, while old or starved cells give no colour reaction with an alkaline 

 solution of a copper salt, which suggests the absence of plastic proteids ". Traces 

 may, however, still be present, since the failure of the reaction is not a sufficiently 

 delicate test to indicate their complete absence, and it is moreover uncertain 

 whether all plastic proteids give the same colour-reaction. Proteids may undergo 

 marked physical modification (as regards solubility, &c.) without any corresponding 

 chemical change, and in this manner the same proteid may be able to subserve 

 a variety of purposes, a plastic substance sometimes being used as building material 

 (Sect. 78). It is moreover certain that a slight difference in chemical constitution 

 may be correlated with a very marked difference in the nutritive value and 

 physiological importance of allied proteids (Sect. 66). Systematic experiments on the 

 nutrition of fungi with various proteids would undoubtedly bring this fact clearly 

 to view, for it has already been shown that mucin, nuclein, &c. afford suitable 

 nutrient material for many of them. Various authors have observed that in starved 

 plants the nuclear chromatin diminishes to a marked extent*. Hence the higher 

 plants are able to reassimilate nucleins, and various nucleoproteids may apparently 

 function as reserve-materials. Similarly in seedlings of Lufinus the amount of 



1 Cf. the analyses by Kbnig, Nahrungs- u. Genussmittel, 1889, and by Ebermayer, Physiol. 

 Chemie, 1883, p. 613. On supposed non-nitrogenous organisms, see p. 388, footnote i. 



a Palladin, Rev. gen. d. Bot., 1896, T. VIII, p. 226, and Zeitschr. f. Biol., 1894, p. 191, where 

 the contradictory results of Weyl, Vines, Green, &c., are quoted. On the storage forms of proteid, 

 cf. Drechsel, Handwbrterb. d. Chemie, 1885, p. 576, and the literature there given. The literature 

 upon gluten substances is given by O'Brien (Annals of Botany, 1895, Vol. IX, p. 171)- Small 

 amounts only of typical albumins are usually present in plants. Griessmayer, Die Proteide d. 

 Getreidearten, Hiilsenfriichte u. Oelsameu, 1897. 



s Sachs, Flora, 1862, p. 297. 



4 Cf. Zimmermann, Morph. u. Physiol. d. Zellkernes, 1896, p. 79. 



