ADJUSTMENT TO WATER 49 



addition, the entrance of soil water into the root-hair cell causes 

 the elastic wall of the latter to stretch. The tendency of the 

 stretched wall to recoil reacts upon the enclosed sap. Since the 

 protoplasmic membrane prevents the escape of the water out- 

 ward, the latter is forcei through into the adjoining cells, where 

 the pull of one membrane is balanced by that of another. 



There is no definite pathway for diffusion between the absorb- 

 ing surface and the fibrovascular bundle. Diffusion may take 

 place in all directions through a tissue. The general direction is 

 determined, however, by the location of the region in which there 

 is a lack of the diffusible substances. In the root the need for 

 the water of absorption increases toward the fibrovascular bundles 

 which are engaged in transporting it upward to the leaves. In 

 similar fashion, the demand for the food materials brought down 

 by the sieve tissue increases toward the outer layers. The gen- 

 eral direction is determined by these two facts, and in the root 

 diffusion is predominantly in a radial direction. It is not cer- 

 tain that diffusing substances follow the shortest route between 

 surface and bundle, but this seems probable. 



Experiment ii. Diffusion in liquids and in tissues. Fill a 10-cc. 

 cylindrical graduate half full of a 5% solution of common salt colored 

 with methyl blue, and with a pipette carefully place 5 cc. of distilled 

 water colored with tropseolin above the salt solution. Note the rate 

 of diffusion. 



Cut from a turnip a strip of tissue a decimeter long and a centimeter 

 wide and deep. Place the strip in a 1% solution of common salt colored 

 with methyl blue. Note the rate of diffusion by cutting off segments 

 of 2 cm. from time to time. 



63. Turgidity. When water is absorbed by the root-hair, 

 the increased pressure within the cell forces the protoplast still 

 more firmly against the wall, and at the same time stretches the 

 latter. The elasticity of the wall leads to a recoil against this 

 force, and as a result the whole cell becomes firm and rigid. 

 The same ])henomenon, which is termed turgidity, takes place 

 in all of the cells of the cortical parenchyma. It is necessarily 

 absent in cells whose walls have lost their elasticity, such as the 

 fibres and vessels of the bundles, and also in cells without pro- 

 toplasm. The turgidity of each parenchyma cell renders the 

 whole cortical region turgid. The turgidity which is thus given 

 to the entire root is further emphasized by the tendency of the 



