SPECIAL SELECTIVE POWER 387 



SECTION 67. Special Selective Power. 



Each plant has a specific preference for particular compounds of 

 carbon, nitrogen, &c., and hence when a variety of carbon-compounds 

 are presented to it, the plant consumes certain of them to a greater extent 

 than others and may entirely neglect some. Thus when supplied with 

 6 per cent, of dextrose and i per cent, of glycerine, Aspcrgillus niger 

 leaves the latter almost intact, whereas the greatest abundance of glycerine 

 does not suffice to protect even a small trace of dextrose from consumption. 

 Similarly glycerine is totally or partially protected by peptone, and butyric 

 acid by dextrose, whereas even when but little acetic acid and a large 

 quantity of dextrose are present the* former is consumed in relatively 

 greater amount, although like glycerine it forms by itself a poor nutrient 

 substance. However much acetic acid may be present, large quantities 

 of dextrose are always consumed and the total supply is ultimately 

 absorbed, which is a sufficient indication of the feeble nutrient value of 

 acetic acid. 



Stereoisomeric bodies may possess very unequal nutritive values : 

 Pasteur observed that Penicillium glancum and Aspergillus nigcr are able 

 to split neutral tartaric acid into its positive and negative optical varieties, 

 and that more of the optically positive tartaric acid is absorbed than of 

 the optically negative. The reverse is the case with a bacterium which 

 develops best upon the optically negative tartaric acid, and when both 

 varieties have the- same nutritive value for a given organism no such 

 decomposition occurs (Aspergillus fumigatus. Bacillus subtilis, &c.). Similar 

 experiments upon other Stereoisomeric substances are given by Pfeffer 1 . 



It is only to be expected that the result obtained should vary according 

 to the plant and to the nutrient mixture employed, but the causes of the 

 differences observed can only be ascertained when a complete knowledge 

 has been gained of all the different factors concerned. It is however possible 

 that with a mixture of, dextrose and acetic acid the latter is assimilated, 

 because it satisfies certain of the plant's requirements more readily than 

 does the better nutrient substance : thus certain fungi arc able to oxidize 

 oxalic acid energetically, although the energy thus obtained apparently 

 cannot be utilized for growth. .Hence the importance of a substance to 

 a plant is not to be measured solely by the amount of growth which it 

 induces, nor does it depend on its chemical and mechanical properties, for 

 a given substance may, in virtue of its special qualities and relative amount, 



1 Pasteur, Compt. rend., 1858, T. XLVI, p. 617, and 1860, T. LI, p. 298 ; Pfeffer, Jahrb. f. wiss. 

 Bot, 1895, Bel. XXVIII, p. 206. Full details and a summary of the literature are given here. 



C C 2 



