90 



BOTANY 



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 be- 

 low. Within two hours the water had risen in the tube as shown in the right-hand figure. 



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 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 (as we shall see later) it is passed over 

 to the tubes of the fibrovascular bundles and started up the 

 stem. The pressure created by this process of osmosis is suffi- 

 cient to send water up the stem to a distance, in some plants, of 

 twenty-five to thirty feet. Cases are on record of water having 

 been raised in the birch a distance of eighty-five feet. How water 

 gets to the summits of tall trees is a problem which we shall dis- 

 cuss in a later chapter. 



Physiological Importance of Osmosis. — It is not an exaggera- 

 tion to say that osmosis is a process not only of great importance 



