QUANTITATIVE SELECTIVE POWER 125 



methyl-blue. Moreover, the plant itself may produce an acid and remove 

 the dye, as is actually the case in Spirogyra^ while any external agencies 

 which induce a formation of acid will also lead to an excretion of the 

 absorbed pigment substance. 



In similar circumstances a weaker acid may completely decompose 

 the salt of a stronger acid, provided the partial reaction is rendered 

 progressive by the continued removal of one of the products of the 

 reaction, by diosmotic or other means. This is in complete accordance 

 with chemical laws, and reactions of this character play a most important 

 part in metabolic processes and in the regulation of metabolic activity l . 

 Since actions of this character may be combined with other factors or 

 form part of a stimulatory chain, they may be utilized by the organism 

 in the most varied manner to attain very different ends. Moreover, the 

 living cell is a complex of organs or separate laboratories, so to speak, 

 in which special powers and structural peculiarities are inherent, while 

 the minute size of the w r hole and of the parts is of great importance for 

 the continuance of a reaction, since it induces and favours both exchange 

 in general and the removal of any given product in particular (Sects. 

 12, 93). Bearing in mind the extremely large surface area of a bacterium 

 or yeast-cell relatively to its bulk, no especially active diosmosis is 

 necessary to permit of the marked exchanges which take place during 

 fermentative activity (Sect. 102). The activity of diosmotic exchange may 

 how r ever be influenced by physical factors to a considerable extent. Thus 

 the greater the difference in concentration between the fluids separated 

 by the diosmotic membrane, the more rapid imbibition and diosmosis 

 will be, for it is by this difference of potential that diffusion and diosmosis 

 are induced and regulated. 



In order to allow rapid diosmotic exchange it is therefore of great 

 importance that the substance after passing through the membrane shall 

 either be carried further into the interior of the cell during endosmosis, 

 or removed by the surrounding fluid during exosmosis. This is more 

 rapidly attained by moving currents than by unaided diffusion, for the 

 latter does not suffice to satisfy the requirements of the cell when 

 food- materials must be extracted from a large quantity of air or from 

 a dilute solution. Common salt is a substance capable of rapid diffusion, 

 but by diffusion alone it requires a period of 319 days to transfer a milli- 

 gramme of salt from a 10 per cent, solution through a distance of i mm. 

 To attain the same result in the case of egg-albumin, a period of about 



1 Pfeffer, Osmot. Unters., 1877, p. 163 ; Unters. a. d. Bot. Inst. z. Tubingen, 1888, p. 293 ; Zur 

 Kenntniss d. Oxydationsvorgange, 1889, p. 463 (cf. Sect. 93). For chemistry see Ostwald, Grundriss 

 d. allgem. Chemie, 1890, 2. Aufl., p. 316, and Loth. Meyer, Die mod. Theorien d. Chemie, 1884, 

 5. Aufl., p. 482. Here a comparison between the combining avidities of different acids is given 

 (pp. 508, 517). 



