230 THE TEACHING BOTANIST 



absorbed by the membrane in virtue of an adhesive attrac- 

 tion between them, but is robbed from the membrane by a 

 stronger adhesion between the dissolved substance and the 

 water, and the limit of the pressure that can be exerted in 

 osmosis would be the limit of this adhesion. The more 

 generally accepted, and probably more nearly correct, view 

 is based upon the fact that the osmotic pressure that can 

 be exerted by any solution is exactly that which the dis- 

 solved substance would exert if converted into a gas confined 

 in the same space at the same temperature. Hence the 

 substance is supposed to be in a compressed gas-like con- 

 dition, constantly exerting expansive pressure, but limited by 

 the boundaries of the liquid in which it is dissolved. Thus 

 the latter is tending always to expand, and hence it will 

 easily absorb any liquid offered to it, as from a wet mem- 

 brane, and this allows it to expand, and hence to rise in a tube, 

 etc. Therefore, when there is a continuous supply of water, 

 as in our osmometer, there will be a steady rise of the solu- 

 tion until the limit of the gaseous expansion of the dissolved 

 substance has been reached ; and hence, also, if this liquid be 

 confined (as in the Pfeffer's artificial cell, particularly well 

 described in Goodale's " Physiology," which the teacher should 

 carefully study in this connection), it will exert pressure upon 

 a gauge. Of course, the energy enabling the gas or dissolved 

 substance to exert its pressure is derived from heat in the 

 atmosphere. The marked difference between the osmometer 

 made by the diffusion shell and the root hair, in that the 

 former allows some of the sugar to pass out, while the latter 

 does not, must be emphasized. In the root hair there is not 

 only a membrane comparable with the parchment, viz. the 

 cellulose wall, but an additional one, a lining film of pro- 

 toplasm, which in the root hairs (but not always in other 



