492 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



hydroquinone, phloroglucin, &c., but except in the case of quinic acid most of 

 these afford very poor food-material l , and commonly exert a poisonous action 

 even when only slightly concentrated. Similarly, owing to the liberation 

 and accumulation of the aromatic compounds such as hydroquinone, from 

 arbutin, &c., an injurious effect is commonly produced when plants are 

 fed with glucosidcs, although so long as a sufficient supply of sugar is 

 present glucosides are rarely attacked. Aromatic substances do not usually 

 seem to serve as food or as storage material, and it is still doubtful whether 

 tannins may be used for such purposes. The latter may form as much as 

 6 per cent, of the total weight of a turgid plant 2 , and a few other 

 benzene derivatives, including glucosides, may be produced in large amount 

 by certain plants. Tannins, phloroglucin, and apparently all aromatic sub- 

 stances which accumulate to any extent, are contained in solution in the 

 cell-sap, so that their presence does not injuriously affect the protoplast. 

 Such substances often afford a certain protection against the ravages of 

 animals by reason of their taste and poisonous character, and in all cases 

 where tannins, glucosides, bitter principles, and any other aromatic substances, 

 such as certain pigments, &c. (Sect. 88), subserve biological functions 3 , they 

 are retained as permanent aplastic products of metabolism. Many of these 

 bodies undergo alteration when the cell dies 4 ; this may in some cases be 

 of importance to the plant, as, for example, in the formation of duramen, 

 which is largely due to the changes occurring in the tannins and other 

 substances which infiltrate the walls of the wood-vessels (Sect. 84). Similarly, 

 the presence of aromatic derivatives may hinder the decomposition of proteids 

 in dead internal tissues. 



Tannins and glucosides are undoubtedly produced for definite purposes, 

 and are not mere by-products produced under all circumstances. In 

 sweet almonds the formation of amygdalin, which characterizes bitter 

 almonds, is suppressed, and, similarly, Loew and Bokorny have observed 

 the cessation of the production of tannin in Spirogyra, and Ashoff the 

 same in Phascolus multiflorus when grown in a nutrient solution containing 

 no chlorides 6 . Pfeffer 6 has shown that certain plants are not injured when 

 the whole of the tannin present in the living cell is precipitated by means 



1 Cf. the works of Nageli and Reinke quoted in Sect. 70. Fungi liberate gallic acid from tannin 

 (Miintz, Compt. rend., 1877, T. LXXXIV, p. 956), but this occurs externally, for they usually contain 

 no gallic acid. 



a G. Krans, Grundlinien z. Physiol. d. Gerbstoffes, 1889, p. 22 ; J. af Klercker, Studien iiber d. 

 Gerbstoffvacuolen, Stockholm, 1888. 



3 Stahl, Pflanzen u. Schnecken, 1888, pp. 32, &c. ; Biisgen, Beobacht. liber das Verhalten d. 

 Gerbstoffe i. d. Pflanzen, 1889 (Jenaische Zeitschr. f. Naturw., Bd. xxiv); Dryer, Bot. Centralbl., 

 1893, pp. 53, 83; Ludwig, Biol. d. Pflanzen, 1895, p. 208. 



4 Pfeffer, Oxydationsvorgange in lebenden Zellen, 1889, p. 49. 



8 Loew u.Bokomy, Biol. Centralbl., 1891, Bd. XI, p. 8; Ashoff, Landw. Jahrb., 1890, Bd.xix, p. 127. 

 Pfeffer, Unters. a. d. Bot. Inst. z, Tubingen, 1886, Bd. II, p. 197; BUsgen, I.e., p. 8. 



