332 PLANT PHYSIOLOGY 



contracts, scattering spores as from a catapult. The whole 

 process may be readily observed under a microscope. The force 

 necessary for such a compression of the cell walls in the annulus 

 was determined by immersing them in highly concentrated 

 solutions of various substances. It proved to be of the magni- 

 tude of 350 atmospheres. 



This immense force of cohesion of the molecules is more than 

 sufficient to raise water to the top of the highest tree, as the 

 pressure of a water column 100 m. high is equal to but 10 atmos- 

 pheres. Hence, the ascent of water in a tree may be represented 

 in the following way. Near the top is the evaporating leaf 

 parenchyma, in whose cells there develops a suction tension of 

 many atmospheres. These cells draw water from the vessels 

 of the vascular bundles, which permeate the whole leaf. Conse- 

 quently, a pull on the water in these vessels is produced, which is 

 transmitted to the vessels of the stem and roots. The water in 

 the wood appears to be suspended to the cells of the leaf paren- 

 chyma. But in the root tips are also parenchyma cells, which, 

 with a sufficient amount of water in the soil and with a slow 

 utilization, support, as it were, the water threads suspended 

 from the leaf cells. Thereby the strain on the leaf parenchyma 

 is reduced, and the water threads are even pushed upward with 

 considerable force. With lack of w^ater in the soil and with 

 simultaneous intensive transpiration, as, for instance, on hot 

 summer days, the supply of water from the root cells cannot 

 keep up with the loss of water from the leaves. In this case, the 

 suction tension of the leaves is transmitted through the columns 

 of water under high tension in the vessels, even as far as the root 

 cells. This scheme has been worked out in detail by Dixon 

 (1901). Since it attributes a conspicuous role in the ascent of 

 water to the forces of cohesion, it is termed the '* theory of 

 cohesion." 



The water columns ffiling the vessels tend to increase in length 

 and to decrease in diameter, like a stretched rubber tube. This 

 tendency would soon cause their breaking into separate drops, 

 which is actually observed when water falls in a thin jet, if they 

 were not contained within the walls of vessels to which they are 

 closely attached by the force of cohesion. When water is under 

 a very high strain in the vessels, the walls of the vessels will be 

 drawn inward, and the diameter will decrease. This transverse 



