THE COHESION OF WATER THEORY 



233 



variation in water content (Fig. 62). A minimum water content is attained 

 during middle or late summer and the trunks gradually fill up during the 

 fall and spring. Often there is a temporary shrinkage in their water content 

 during the winter months but the main period of diminishing trunk water 

 content occurs in the spring and summer. Some authorities even speak of 

 a seasonal rise and fall in the "water table" of a tree trunk, and consider that 

 intercellular spaces as well as cells become occupied Mnth water during this 

 process (Priestley, 1930). "Root pressure" undoubtedly accounts at least in 

 part for such seasonal replenishments of the store of water in tree trunks. 



Fig. 62. Seasonal variation in the water content of the trunks of the trembling aspen 

 {Populus iremuloides) . Data of Gibbs (1935.) 



The living cells of the xylem probably play an active part in this process. It 

 is possible that there may be slowly ascending streams of water in the living 

 cells of the xylem which operate entirely independently of the water columns 

 in the vessels or tracheids. Hence the same portions of the xylem which at 

 times contain gases may at other seasons be occupied by water. The trunks 

 of many conifers show no marked seasonal variations in water content; this 

 is possibly correlated with the lack of appreciable root pressures in such species. 



The Cohesion of Water Theory. — The leading advocate of this theory 

 has been Dixon (191 4, 1924) who has performed much of the experimental 

 work upon which it is based. A number of other workers, notably Askenasy 

 (1897) and Renner (1911, 1915), have also contributed important theoretical 

 and experimental evidence in support of this theory. 



INIolecules of water, although ceaselessly in motion, are also attracted to 

 each other by strong cohesive forces. In masses of liquid water the existence 

 01 such cohesive forces is not obvious, but when water is confined in long 



