NUTRITIVE VALUE OF DIFFERENT CARBON COMPOUNDS 385 



solutions. Thus certain fungi develop slowly upon dilute solutions of carbolic 

 acid, quinine, morphia, &c., and stronger solutions might form good nutrient media 

 were it not for the poisonous influence which they then exercise. The accumulation 

 of excrete products and the changes produced in the nutrient fluid commonly retard 

 further development. All these factors may influence the experimental results 

 obtained, whereas the disturbing effects of competition may be avoided by the use 

 of pure cultures (cf. Sect. 92). 



Apparently trifling circumstances have often a marked influence upon growth 

 and development, and in many plants the commencement of the latter is due to 

 the action of specific chemical stimuli (Sect. 64). Further, as Raulin found, a small 

 dose of zinc or manganese produces an increased crop of fungus, and according to 

 Richards, the same effect is given by almost any poison, although, as might be 

 expected, with stronger doses growth is retarded '. This reaction to stimulation by 

 accelerated growth is apparently due to an attempt on the part of the plant to 

 counteract an injurious influence as far as possible by an increased vital activity, 

 and similar phenomena may follow mechanical injury 2 . 



Economic coefficient. Hitherto the nutritive value of a carbon-compound has 

 been estimated by the rapidity of development, but this affords no criterion as to 

 the economic coefficient, that is, the weight which is produced by a consumption of 

 100 parts of the nutrient material 3 . This economic coefficient, like the respiratory 

 quotient, is variable, and when the temperature is raised above the optimum for 

 growth a larger amount of food-material is consumed in the increased respiratory 

 activity without any equivalent growth (Sect. 95). Under otherwise similar conditions 

 the economic coefficient is smaller for an innutritious carbon-compound involving 

 slower growth, than for a more suitable one. If, however, when two substances are 

 jointly supplied, one of them is consumed mainly in respiration, the economic 

 coefficient of the other will be correspondingly increased. The value of a substance 

 is not dependent simply upon the amount of energy which may be derived from its 

 combustion, nor is there any direct connexion between the nutritive value of 

 a substance and the store of potential energy which it contains. 



Methods. A general account of the mode of cultivation, as well as of the 

 precautions necessary to obtain pure cultures, is given by Hueppe 4 . The cultures 

 are best performed in small flasks containing 20-200 cc. of sterilized nutrient 

 fluid, the mouths of the flasks being closed by plugs of cotton wool. The medium 

 must be neutral or feebly alkaline for most bacteria, and kept in this condition if 

 necessary by the addition of sodium carbonate, or of chalk, whereas cultures of 

 fungi should be made feebly acid by means of phosphoric acid. 



If no organic nitrogen-compounds are necessary, the addition of 0-5 to 2 per cent, 

 of one of the saline mixtures mentioned in Sect. 73, together with 2 to 15 per cent. 



1 Raulin, Ann. d. sci. nat., 1869, v. &., T. xi, p. 252; Pfeffer, Jahrb. f. wiss. Bot., 1895, 

 Bd. xxvin, p. 238 ; Richards, ibid., 1897, Bd. xxx, p. 665. 



2 Cf. Pfeffer, 1. c., p. 238. Cf. also Sect. 92, and on increased respiration Sect. 104. 



3 See Pfeffer, 1. c., p. 257 ; also Kunstmann, Uber das Verhaltniss zwischen Pilzernte u. ver- 

 brauchter Nahrung, 1895. 



* Hueppe, Die Methoden d. Bacterienforschung, 1891, 5. Aufl., &c. On anaerobes, cf. Sect. 98. 



PFEFFER C C 



