How Plants Draw in Various Materials 183 



since it merely concerns the explanation of osmotic pressure, 

 and is not essential to the integrity of our subject. Now a 

 very remarkable and important point about osmotic pressures 

 is this, that in general they are the same in amount as would 

 be given by the respective substances if converted into gases 

 at the same volume, temperature and pressure. This carries 

 the implication that osmotic pressures and gas pressures, be- 

 ing the same in amount, are the same in kind, the dis- 

 solved substance being practically a gas, and it, not the liquid, 

 exerting the pressure. But while this explanation is satisfactory 

 for most of the phenomena, it meets with the physical difficulty 

 that the closely packed water molecules must prevent that free- 

 dom of back and forth movement upon w^hich a gas pressure 

 depends. Accordingly a second explanation has been given, 

 really an old one revived, which finds the source of the pressure 

 in an adhesive attraction between the molecules of the dissolved 

 substance and those of the water, whereby the former draw all 

 of the latter around them, and take more from the membrane 

 (which easily recoups itself from the outside supply); and thus 

 the solution swells and the pressure is obviously exerted by the 

 substance and liquid in combination. Or, one can express the 

 same thing by imagining that the molecules of the dissoh^ed 

 substance act Uke the micellae of the membrane and absorb 

 water (from the latter) by imbibition, with only this difference 

 that the adhesion between substance and water is stronger for 

 all distances than the cohesion of the substance for itself. And 

 still a third explanation is possible, namely, that the spaces 

 between the suspended molecules of the dissolved substance act 

 like excessively fine passages along which the water passes forcibly 

 by an extremely refined capillarity, — in which case the water, and 

 not the substance, exerts the pressure. And if it seems that 

 the correspondence between osmotic pressures and gas pressures 

 must be conclusive for the first explanation against the others, it 

 is to be said that this is not necessarily true, for the properties 



