138 



GENERAL BOTANY 



Glass tube 



Cork 



and the water in which it is dissolved is termed a solvent. The 

 solutes are usually said to be of a certain concentration, which 

 means the relative amount of the substance dissolved in the 

 water (the solvent) in a unit of volume. 



In the experiment, therefore, if the salt and water are assumed 

 to occupy equal portions of the space in the diffusion shell, the 

 salt is of greater concentration inside the shell than outside in 



the distilled water, 

 where it would be 

 nil; and the water 

 is greater in amount 

 per unit of space 

 in the jar than 

 inside the shell. 

 Neglecting for the 

 moment the physi- 

 cal explanations for 

 the movements of 

 the solvent (water) 

 and the solute (salt), 

 we have seen that 

 each tended to flow 

 from a point of 

 greater to a point 

 of less concentra- 

 tion, and this result 

 may be taken as a 

 common law, or tendency, of substances of a liquid nature and 

 of different concentration separated by a membrane. Since, 

 however, the parchment membrane in the experiment allowed 

 the water to pass in freely, being permeable to it, and hindered 

 the outgo of salt, the result was a great increase in the volume 

 of the water in the parchment tube, and a corresponding increase 

 of pressure (called osmotic pressure) which tended to overdis- 

 tend the tube. As the glass tube furnished an easy exit for the 

 water, and a relief, as it were, from the osmotic pressure in the 

 parchment tube, the water rose against gravity, thus giving rise 



Solvent 



FIG. 67. Experiments in osmosis and root pressure 



a, a diagram illustrating an experiment in osmosis ; 6, a 



diagram of an experiment illustrating the exudation of 



water from a cut stem 



