THE ASCENT OF SAP 265 



to meet the demand in times of excessive transpiration. The 

 depletion of this reservoir results in the " negative pressures" 

 observed. Such conceptions, although vague, seem to infer that 

 the living cells at the various levels in the tree act like a system 

 of relay pumps, forcing the water from one level to the next. That 

 living cells are quite unnecessary for the rise of sap, at least in 

 small plants like Cyperus, has been shown by J. B. Overton (1911), 

 who killed all the cells of the stem for an inch or more and found 

 that the transpiration stream was not affected by this treatment. 

 That such cells are not necessary is hence shown conclusively, 

 but this does not prove that living cells when present may not 

 assist the rise of water. Overton, however (1926), thinks that the 

 living cells in contact with the conducting elements may actually 

 be a hindrance, since they may form tyloses and thus block the 



vessels. 



Cohesion Theory.— The explanation now viewed with the most 



favor by physiologists is that of Dixon (1909). According to his 



theory the evaporation of the water from the leaves pulls the 



water in a continuous column through the tree. The water is 



like a rope which is pulled up as the upper end vaporizes. This 



theory consequently rests upon the cohesive forces in water, and 



the question becomes simply one of whether the water rope, which 



is 400 feet long in the tallest trees, will hang together without 



breaking. Physicists have estimated the cohesive force of water 



to be 10-150 atmospheres, while Dixon asserts that the cohesive 



force of the sap is at least 100 atmospheres. Ten atmospheres, 



or the minimum estimated, would be about all that is required 



and if we add another 10 for internal friction within the cells of 



the wood, we find that 20 atmospheres would seem to be plenty. 



With 100 atmospheres present as claimed by Dixon, there seems 



to be an unusually large safety factor! 



Is there an evaporation force which can lift the column of 

 water? is another question which arises. Bohm (1893) in a simple 

 experiment had already shown that the force of evaporation was 

 large. A leafy twig was sealed into the top of a glass tube filled 

 with water and the lower end was allowed to dip into a vessel of 

 mercury. As the water evaporates it is replaced by the water in 

 the tube, and the mercury from below is then drawn up into the 

 tube to replace the water. Bohm found the mercury rose 85-90 cm., 

 which is thus seen to be more than an atmosphere. 



