90 



ROOTS AND THEIR WORK 



brane and mingle with each other is called osmosis.^ The method 

 by which the root hairs take up soil water is exactly the same pro- 

 cess. It is by osmosis. The white of the egg is the best possible 

 substitute for living matter; it has, indeed, almost the same 

 chemical formula as protoplasm. The celloidin membrane sepa- 

 rating the egg from the water is much like the dehcate membrane 

 and 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 water, the density 

 of the cell sap is less- 

 ened, 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 



^ For an fxcellent elementary discussion of osmosis see Moore, Physiology of 

 Man and other Animals. Henry Holt and Company. 



A potato osmometer. The lower end of the potato 

 was cut off and the remainder peeled for about one 

 third of its length. A hole was bored to within 

 three fourths of an inch of the cut end ; a small hole 

 was bored at the side of the potato. In the latter 

 was inserted a small L-shaped tube, the lower end 

 being vaselined to make it air-tight. Sugar was 

 then placed in the hole at the top and a cork inserted ; 

 water was poured into the dish below. Within two 

 hours the water had risen in the tube, as shown in 

 the right-hand Figure. 



