ABSORPTION OF WATER 



137 



were, a self-regulating osmotic apparatus. The suction tension, 

 originating in the leaves is finally transmitted through a series of 

 mechanisms to the root system, where it creates a water deficit 

 which is necessary for the initiation of suction. 



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

 oration, this saturation deficit may be called "passive suction." 

 The root system possesses a means of creating this deficit in its 

 cells in a purely physiological way, 

 because of the water-expelling activity 

 of the cells of roots. The water is 

 driven in a definite direction, namely, 

 through the cells of the cortex into 

 the vessels of the xylem, which are 

 situated in the central cylinder. The 

 course of the water current ascending 

 in the root may be represented by 

 the following diagram (Fig. 52). Into 

 cell A, provided with a root hair, the 

 water is absorbed from the soil and 

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

 reaches cell L, adjoining vessel M. 

 Here it is driven into the vessel and 

 ascends the stem. 



To observe the active pressure of 

 the root cells, a plant is cut above 

 the soil. To the cut stump a rubber 



tube is fastened ending in a glass Fig. 52.— Path of water from the 

 , _,. , j ,i i root hair A to the vessels of the 



tube. The water excreted through root M {after Pries aey). 



the cut surface accumulates and 



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

 placed under it, the water exuding from the root system may be 

 gathered and analyzed (Fig. 53). This phenomenon has been 

 called the "bleeding" of plants. The force that drives the 

 water into the vessels is termed "root pressure." To measure 

 this force, a mercury manometer is connected to the cut stump 

 of the plant. 



The phenomenon of bleeding is exhibited by various plants in 

 different degrees. In some, as Fuchsia, nettle, sunflower, it is 

 readily detected. In others, for instance the conifers, it is almost 

 imperceptible. Bleeding shows also seasonal variations. It is 



