54 BOTANY: PRINCIPLES AND PROBLEMS 



sap-cavity of the root-hair, where sugar and other materials are 

 dissolved. In obedience to the law of osmosis, therefore, water 

 will pass from the soil solution through the cytoplasmic mem- 

 branes of the root-hair and into its sap-cavity. This flow of water 

 will continue so long as there is a difference in total density 

 between soil-solution and sap-solution. Of course if the soil 

 becomes dry and the film around each particle grows so thin that 

 the surface attraction of the particle equals the osmotic attrac- 

 tion of the root-hair solution, the flow will necessarily cease; and 

 if this condition occurs throughout the whole soil mass, the plant 

 will suffer from drought. 



Salts and other substances in the soil-solution diffuse through 

 the root-hair membranes quite independently of the passage of 

 water, and the rate at which they enter depends upon the factors 

 which we have above considered. Any substance which is in 

 greater concentration in the cell-sap than in the soil-water, and to 

 which the membranes are permeable, will of course, diffuse 

 outwardly into the soil; but except for carbon dioxide, which is 

 given off in considerable quantities as a product of respiration in 

 the root, there seems to be comparatively little loss of material 

 from the plant in this manner. 



As water is taken into the sap-cavity of the root-hair, the 

 solution there becomes less dense ; and the first cell of the cortex 

 is consequently able in turn to withdraw water osmotically from 

 the root-hair cell. The second row of cortical cells may now 

 withdraw water from the first, and this process will continue until 

 the water reaches the central cylinder. The water-ducts here, 

 however, are nothing but dead shells, their living cytoplasm 

 having disappeared as soon as the thick cell walls were completed. 

 They are filled with water, and it is hard to understand why 

 water should move into them from the cells of the cortex rather 

 than in the reverse direction. In the innermost layer of cortical 

 cells, a considerable pressure is probably developed by osmosis, 

 and water may simply be squeezed through the cytoplasm and 

 into these ducts. We know, at any rate, that water is forced up 

 through the ducts under a good deal of pressure. This root- 

 pressure may be measured by a gauge attached to an opening in 

 the stem. As to what causes water to rise to great heights in 

 the trunks of trees we shall speak later; but root-pressure is 

 apparently only one of the factors involved. 



