Absoeption axd Tbansmissiox of Solutes 119 



and that different plants absorb different amounts of the 

 same substance. Failure to absorb a solute which is plenti- 

 ful in the external solution may be due to either one of two 

 causes : either the protoplasmic membrane is impermeable 

 to that substance, or its diffusion tension within and without 

 are equal. If a substance is not used in metabolism, it may 

 simply remain in the cell sap, at the same concentration as 

 is the surrounding medium, or it may be precipitated or 

 condensed in the cell sap or in the protoplasm, and thus con- 

 tinue to accumulate. An example of this last possibiUty is 

 met with in the case of the storage of carbohydrates. Sugar 

 diffuses into a cell and is there polymerized into insoluble 

 starch. This process continually removes the sugar from 

 solution, so that inward diffusion must continue as long as 

 starch can be formed, and as long as sugar is plentiful out- 

 side. Another example of accumulation is the case cited by 

 MacDougal, in which large quantities of metallic copper 

 were found in the cells of an oak tree (see p. 69). 



If the substance is used in metabolism, as are XO3 ions, 

 for instance, there must also occur a continuous diminution 

 of the internal diffusion tension of these particles, which 

 can only be met by inward diffusion from without. If a 

 substance is being rapidly used, this inward diffusion will be 

 correspondingly rapid ; if it is but slowly used, absorption 

 wiU be correspondingly slow; and all this adjustment of 

 absorbing power may thxis take place without any change in 

 the permeability of the plastic membranes. It is probable 

 that most cases of selective power are to be explained in this 

 way. Just as there is a great difference in the use of the 

 various absorbed solutes by different plants, so also there must 

 be a corresponding difference in the amounts absorbed. Thus, 

 DemoussT,' using equivalent quantities of several salts, found 



1 E. Democsst. " Absorptioa Elective de quelques filaments miiiSrairx par les 

 plants," CkmpU rend.. Vol. CXXTII (1900), pp. 970-73; cf. P. Boijkgkt, "Sur 

 I'absorption de I'iode par les Tfegfitaux," ibid., VoL CXXIX (1899), pp. 768-70; idem, 

 same title, Bua. Soc. chim. de Faris, Ser. IH, Vol. Will (1899), pp. 40, 41. 



