WATER RELATIONS OF PLANTS 



283 



Arising in the plant as a result of the physical process of 

 evaporation, this suction tension is 

 transmitted down the stem as a ten- 

 sion on the continuous water column 

 in the conducting vessels and to the 

 cells of the root. However, the 

 latter possesses also a purely physio- 

 logical means of forcing water upward 

 through the conductive system. 

 This is the phenomenon called ''root 

 pressure," which drives the water 

 through the cells of the cortex and 

 up into the vessels of the xylem. 

 The path of the water ascending in 

 the root may be represented by the 

 adjacent diagram (Fig. 83). Into 

 cell A, provided with a root hair, the 

 water is absorbed from the soil and 

 moved on to the cells B, C, etc., until 



it reaches cell L, adjoining vessel M. ^^^ 33 _p^^j^ ^^ ^^^^^ ^^^^ 

 Here it is driven into the vessel and the root hair A to the vessels of 

 ascends the stem. ^^^ ^^^^ ^^ ("-^^^^ Priestley). 



To observe root pressure, a plant is cut above the soil. A 

 rubber tube, ending in a glass tube is fastened to the cut stump. 



Fig. 84. — Exudation of sap from a cut as a result of root pressure (after Molisch) . 



The water excreted through the cut surface accumulates and 

 rises in the glass tube. If the latter is bent down and a vessel 



