FACTORS INFLUENCING HIBIBITION 



III 



sures, after which the amount of water which had been imbibed was deter- 

 mined. The coats of these seeds are permeable to water, but virtually imper- 

 meable to these solutes. Were it not for this latter fact complicating effects 

 due to the specific influences of the ions emploj'ed, as described in the next 

 section, might invalidate the results of experiments of this type. The results 

 of such a series of determinations are shown in Table 1 7. 



TABLE 17 IMBIBITION OF WATER BY COCKLEBUR SEEDS IMMERSED IN SOLUTIONS OF 



DIFFERENT OSMOTIC PRESSURES (dATA OF SHULL, I916) 



The general principle shown by the results of these experiments is that 

 with increase in the osmotic pressure of the solution in which the seeds are 

 immersed, the amount of water held by imbibition per unit of dry weight at 

 the equilibrium point decreases. Since at equilibrium the diffusion pressures 

 of the water in the imbibing substance and in the surrounding liquid must 

 be equal the basis for this osmotic effect upon imbibition is evident. If the 

 diffusion pressure in the solution is relatively low (high osmotic pressure), 

 the diffusion of less water into the imbibing substance is necessary to raise 

 the diffusion pressure of the water in it to a value equal to the diffusion 

 pressure of the water in the circumambient liquid, than if the diffusion pres- 

 sure in the surrounding liquid is high. The relation between the osmotic 

 pressure and the amount of water imbibed is not, however, a strictly propor- 

 tional one. At the lower end of the range of concentrations employed, an 

 increase of a few atmospheres in osmotic pressure causes a decrease in the 



