510 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



, always present, and in its absence the ferment exerts only a very feeble oxidizing 

 power, such as is also possessed by oxide of manganese, whereas their conjoint 

 action induces twenty to thirty times more active oxidation. Laccase has also been 

 detected in various fungi, such as Russula, Lactarius, and Boletus. It causes the 

 oxidation of a chromogen present in the expressed sap of these plants into blue, red, 

 or black pigments. Bertrand ' has also isolated another oxidase ' tyrosinase ' from 

 Russula and other fungi, from the dahlia, and from beetroots. It rapidly oxidizes 

 tyrosin into a black pigment, but exercises no action upon pyrogallol or hydroquinone. 

 Tyrosinase is destroyed at from 50 C. to 60 C., and hence laccase may be separated 

 from tyrosinase by heating to 70 C. The action of tyrosinase is not hindered 

 by the presence of 50 per cent, of ethyl- or methyl-alcohol. It is doubtful whether 

 the 'oenoxydase' which frequently causes the soluble pigment of red wines to 

 be precipitated is a special ferment or is identical with laccase 9 . In many plants 

 chromogens occur whose post-mortem oxidation may or may not be due to oxidase 

 enzymes. The hypothetical 'leptomin' which Raciborski (Ber. d. D. Bot. Ges., Aug. 

 1898) finds to be commonly present in the living elements of the conducting 

 channels of many plants is also apparently an oxidase ferment. Its presence may 

 be detected by the blue colouration given on testing with guiacum solution and 

 hydrogen peroxide, but according to Griiss this reaction is due to the presence of 

 diastase. A similar reaction may also be given by young tissues but not after 

 immersion in alcohol, whereas the oxidase ferments included under the head of 

 ' leptomin ' retain their characteristic properties under such treatment, although the 

 latter may be weakened. Haemoglobin and haemocyanin are bodies which have 

 a similar power of inducing or accelerating oxidation.] 



SECTION 92. External Influences. 



Metabolism like vital activity is dependent upon the external conditions, 

 and both its activity and character may be modified according to existing 

 circumstances. Detailed experiments are, however, as a general rule neces- 

 sary to detect such changes, for it is seldom that the products of metabolism 

 are so immediately evident as they are in the case of pigment substances. 

 The amount of consumption or excretion enables the course of respiration 

 or fermentation to be approximately determined, and similarly changes in 

 the rate of growth, in the production of heat, in the power of movement, all 

 afford more or less marked indications of an alteration in metabolic activity. 



The different functions are, however, not always influenced to the same 

 extent and in the same manner : thus respiration continually increases 

 as the temperature rises (Sect. 104), whereas growth and the constructive 

 metabolism associated with it diminish or cease beyond a certain optimal 

 temperature. Similarly the production of pigments, poisons, enzymes, &c. 



1 Bertrand, Compt. rend., 1896, T. cxxin, p. 463. 



1 Martinand, Compt. rend., 1897, T. cxxiv, p. 512; Bouffard, ibid., p. 706; Cazeneuve, ibid., 

 pp. 406, 781. For further literature, see Green, Science Progress, 1898, Vol. vn. p. 253. 



