186 



TEXTBOOK OF PLANT PHYSIOLOGY 



rise of the mercury. Indirect measurements, based on the deter- 

 mination of suction tension in separate cells of a leaf, have shown, 

 however, that a force of several atmospheres is attained. Thus, 

 in the plant at the lower and the upper ends of the vascular system 

 are two forceful pumps, the one below lifting water, the other 

 pulling it. These two mechanisms provide the plant with a con- 



Fig. 83. — Transpiration pull shown by rise of mercury in the tube (after Molisch). 



tinuous water current. The upper terminal device shows a 

 number of advantages over the lower one. It is able to develop a 

 much greater force, it is self-regulating, and it works at the expense 

 of radiant energy, which heats the leaf and increases transpiration. 

 The lower mechanism works at the expense of nutritive substances 

 supplied by the leaves. It is little wonder, therefore, that the 

 upper equipment is more efficient than the lower, and that most of 



