326 



PLANT PHYSIOLOGY 



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gators, therefore, tried to explain this phenomenon by capillarity. 

 The source of energy causing the capillary rise of fluids in narrow 

 tubes is the force of adhesion between water and the vessel wall, 

 shown by the concavity of the upper meniscus. But the plant 

 vessels are completely filled with water. No meniscuses are 

 found in them. Moreover, even if such meniscuses existed, 

 with an average diameter of the vessels of 0.1 mm., water would 

 be able to rise by capillarity only about 30 cm. In trees, the 



water has to be lifted many meters. Hence, 

 the theory explaining the upward movement of 

 water in the plant by capillarity was soon 

 discarded. 



The water-conducting tissues being passive 

 elements, the active movers of the water current 

 are the living cells that adjoin the vascular 

 system in its upper and lower end. At the base 

 of the plant, this work is performed by the cells 

 of the root parenchyma, which are in close con- 

 ° showing the ^^^^ with the vessels ; while above, it is accom- 

 arrangement of the plished by the cells of the leaf parenchyma, 



terminal mechan- i • v j j.u j.* r i u Ji 



which surround the tips oi vascular bundles 

 (Fig. 104; also in detail in Fig. 102). As a 

 result of the activity of these cells, the water 

 current moves through the dead tissue of the 

 wood. 



The work of the lower terminal mechanisms 

 has already been discussed in Art. 65. It has 

 been mentioned that the root-parenchyma cells 

 are able to pump water into the vessels with a force of 2 to 3 

 atmospheres. Hence, with low transpiration the surplus of water 

 may be forced out of a plant in the form of drops. The impor- 

 tance of this lower motive force, however, is not nearly so 

 significant as is that of the upper terminal motor, the transpiring 

 cells of the leaf parenchyma. 



That transpiration is a sufficient source of energy to move the 

 water supply of the plant may be seen from the well-known fact 

 that portions of a plant, when cut off and placed in water, preserve 

 under favorable conditions their freshness for a long time, using 

 up a considerable amount of water. The mechanism of the 

 transpiration pull is not so very complicated after all. It is 



isms of the water 

 current :leaf and root 

 parenchyma with 

 vessels connecting 

 them: (a) leaf paren- 

 chyma; (6) vessels; 

 (c) root paren- 

 chyma; (d) root hair 

 {adapted from 

 Walter). 



