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TEXTBOOK OF PLANT PHYSIOLOGY 



becomes diluted with water and its osmotic pressure falls, the suc- 

 tion tension rapidly decreases. On the right side of the diagram 

 is illustrated the condition of complete saturation with water, the 

 suction tension having fallen to zero and the turgidity now bal- 

 ancing the entire strength of the osmotic pressure of the cell. 

 Under the usual "average" conditions of a cell, denoted in this 

 diagram by the letter G, a part of the osmotic pressure is balanced 

 by turgor pressure. The other unbalanced part represents the 

 magnitude of the suction tension. 



Fig. 51. — Diagram showing changes of the osmotic value in a cell passing from 



saturation with water to wilting (from right to left), and vice versa (from left to 



right): T, turgor pressure; O, osmotic pressure of the cell sap; S, suction tension; 



G, degree of distention of the membrane (after Hdfler). 



43. Suction Tension and Exudation Pressures of the Root 

 System. Root Pressure, Guttation, and Bleeding of Plants.- 



Land plants of simple structure, for example the mosses and lichens, 

 absorb water through their whole surface. In higher plants, spe- 

 cial organs, the fine tips of roots or rootlets, are provided for this 

 purpose. The cells of this part of the roots show thin, unpro- 

 tected walls. In the majority of cases they are provided with 

 special outgrowths, the root hairs, which greatly increase the 

 absorbing surface of the plant. Root hairs penetrate into the soil 

 capillaries and absorb the water contained therein. 



If the plant did not lose water continually in the process of 

 transpiration, the root cells would soon be saturated with water 

 and absorption would cease. The loss of water by the plant must, 

 therefore, be regarded as the principal process inducing an unsat- 

 urated condition in the plant and promoting the formation of con- 

 siderable suction tension. The plant as a whole presents, as it 



