Conditions and Principles of Absorption 65 



portance, and particularly important in the activities of 

 the nonliving cell-walls. The maintenance of the water- 

 current or water-content of the plant is conditioned by 

 imbibition, but under the conditions of growth the inflow 

 of water into the cells of the root surfaces is effected by the 

 force of osmosis, a mental picture of which is essential to 

 an understanding of absorption phenomena. 



40. Osmosis and diffusion. Osmosis and diffusion 

 as generally understood determine the inflow of water to 

 the root-hair, as well as that of the nutrients or other sub- 

 stances (solutes) in solution. These forces are likewise 

 important in the interrelations existing between cells. 



The fact of diffusion is readily observed. If a crystal of 

 copper sulfate is placed in a tumbler of water, the salt goes 

 into solution, and in time the colored solution diffuses 

 itself equally throughout the water. This diffusion is 

 wholly independent of any convection currents due to 

 changes of temperature, and it is true for all such soluble 

 substances as sugar, common salt, and the like. 



The movement of the particles of the dissolved sub- 

 stances from the region of greater concentration to that 

 of less implies a force, or pressure, which may be termed 

 osmosis, or diffusion tension. 



41. The demonstration of osmotic pressure. The 

 osmotic action of a solute, in water as a solvent, may be 

 conveniently demonstrated qualitatively by a simple 

 experiment in which it is made evident as hydrostatic 

 pressure. A factor not yet mentioned which is involved 

 and which is absolutely essential in this demonstration is 

 a semipermeable membrane, an imbibition membrane 

 which, in this case, permits water to pass through readily, 



