90 



ROOTS AND THEIR WORK 



brane and mingle with each other is called osmosis. 1 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 delicate 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 



A potato osmometer. The lower end of the potato ^Water, the densit y 



was cut off and the remainder peeled for about one f tne Ce ^ sa P is ^ ess ~ 



third of its length. A hole was bored to within ened, and the Cells of 



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



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



was inserted a small L-shaped tube, the lower end 1U a P osltlon to pass 



being vaselined to make it air-tight. Sugar was along their supply of 



then placed in the hole at the top and a cork inserted ; water to thp r>plk TIPYT 

 water was poured into the dish below. Within two 



hours the water had risen in the tube, as shown in tO tnem and nearer to 



the right-hand Figure. 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 



1 For an excellent elementary discussion of osmosis see Moore, Physiology of 

 Man and other Animals. Henry Holt and Company. 



