7G 



SOIL AND ITS RELATION TO ROOTS 



the same process. It is by osmosis. The white of the egg is the 

 best possible substitute for Uviiig matter ; the celloidin membrane 

 separating the egg from the water is much hke the dehcate mem- 

 brane-hke wall which separates the protoplasm of the root hair 

 from the water in the, soil surrounding it. The fluid in the root 

 hair is denser than the soil water ; hence the greater flow is toward 

 the interior of the root hair.^ 



Passage of Soil Water within the Root. — - We have already seen 

 that in an exchange of fluids by osmosis the greater flow is always 

 toward the denser fluid. Thus it is that the root hairs take in 

 more fluid than they give up. The cell sap, which partly fills 

 the interior of the root hair, is a fluid of greater density than the 

 water outside in the soil. When the root hairs become filled with 



The soil particles are each surrounded with a delicate film of water. 

 How might the root hairs take up this water ? 



water, the density of the cell sap is lessened, and the cells of the 

 epidermis are thus in a position to pass along their supply of water 

 to the cells next to them and nearer to the center of the root. 

 These cells, in turn, become less dense than their inside neighbors, 

 and so the transfer of water goes on until the water at last reaches 

 the central cylinder. Here it is passed over to the tubes of the 

 woody bundles and started up the stem. The pressure created 



^ For an excellent elementary discussion of osmosis see Moore, Physiology oj 

 Man and Other Animals. Henry Holt and Company. 



