ENZYMES 503 



most ferments are destroyed at a comparatively low temperature, their hydrolytic 

 action is unable to increase indefinitely as the temperature rises, although the 

 hydrolysing power of such substances as hydrochloric acid may become per- 

 ceptible only at high temperatures, and may increase until the limit of molecular 

 stability is reached 1 . It is, moreover, not surprising that hydrochloric acid 

 decomposes all carbohydrates and glucosides, but enzymes only particular forms 

 of these substances. 



Ferments serve both to digest food-substances and to render them 

 capable of absorption, as well as to prepare them for assimilation by the 

 protoplast. Hence the production and excretion of ferments is commoner 

 among heterotrophic than among autotrophic plants. If the former are 

 supplied with nutriment which can be directly assimilated, the ferments 

 which may still be excreted are no longer of essential importance, and many 

 plants produce ferments only at certain stages of development or not at all. 

 In certain cases the production of ferments ceases partially or entirely under 

 unfavourable cultural conditions, and even in an actively growing plant an 

 abundant supply of the products of enzymatic action may partly or wholly 

 inhibit the formation of the enzyme in question. The latter is an example 

 of a diminution of metabolic activity induced by the action of a stimulus, 

 whereas in carnivorous, and probably many other plants also, the secretion 

 of ferment is excited or accelerated by special chemical stimuli. The 

 excretion of ferment may excite fresh secretion, and the rate of secretion 

 is subject to automatic regulation. 



According to circumstances enzymes may be excreted externally, 

 either continually or only at certain times, or may be always retained 

 within the plant. In the latter case the ferment and fermentable material 

 may be separated from one another, or the possibility of interaction may be 

 prevented owing to one or the other being present in some inactive form or 

 combination (zymogen, &c.). The action of an enzyme may be either aided 

 or inhibited by the presence of other substances, and moreover not only 

 the production but also the transport of ferments is subject to regulatory 

 modification 2 . 



In many cases zymogens exist in the living cells, and when excreted 

 readily decompose or change to form active enzymes 3 , but the metabolic 



1 Tammann, Zeitschr. f. physiol. Chemie, 1892, Bd. xvi, p. 271; Zeitschr. f. physik. Chemie, 

 1895, Bd. XVIII, p. 426. 



2 There do not as yet appear to be sufficient grounds for assuming that a chemical difference exists 

 between translocatory and secretory diastase (Brown u. Morris, Bot. Zeitung, 1892, p. 465). Different 

 extracts of diastases, invertins, &c., possess unequal diosmosing powers. A. Meyer, StarkekCmer, 

 1895, p. 228; Griiss, Jahrb. f. wiss. Bot., 1894, Bd. xxvi, p. 386; Beyerinck, Centralbl. f. Bact, 

 1895' Abth. ii, Bd. I, pp. 223, 228 ; Fermi (1. c. beneath). 



s Cf. Green, Annals of Botany, 1893, Vol. vn, p. 121 ; Frankfurt, Versuchsst., 1896, Bd. xi ,'H, 

 p. 455. The proteids from which enzymes have been artificially produced may have been zymogens. 

 See Schleichert, Die diastat. Fermente d. Pflanzen, 1893, p. 84. 



