142 DIFFUSION AND OSMOTIC PRESSURE 



same principle, but since its original pressure, say s, was 

 much smaller than p, its decrease for the same fall of tem- 

 perature will not be so great as that of the internal solution. 

 This may be shown thus: 



t'p t's 



273 + t^ 273 + t ' 

 when p > s . 



Thus, the internal and external osmotic pressure will be 

 more nearly the same at a low temperature than at a higher 

 one. The two pressures should become equal at absolute zero. 



No measurements have been made to determine whether 

 the decrease in volume of the Spirogyra vacuole is propor- 

 tional to the approach of the external and internal concen- 

 trations toward each other. This should not be a difficult 

 thing to settle. But, as has already been stated (page 75), 

 there is cryoscopic evidence that the extruded liquid is not 

 pure water. 



The identity of the responses obtained by Loeb with 

 Copepods and Polygordius larvae when these were subjected 

 to cold and to high concentrations, has also been noted 

 (page 139). A similar change of tropism occurs among 

 those plant lice which exist in two forms, one winged and 

 the other wingless. The growth of wings in the wingless 

 form can be called forth either by low temperature or by 

 allowing the plants upon which the animals are feeding to 

 dry, thus depriving the latter of water. While in the wingless 

 condition these lice are negatively heliotropic, but upon devel- 

 oping wings they become positively so. Here is a reversal 

 of tropism brought about by withdrawal of water, but this 

 experiment also shows that, although the general protoplas- 

 mic activity may be depressed by this treatment, yet certain 

 special activities (e. g., those involved in wing formation) 

 may be accelerated. 



