276 The Physiology of Plants BOOK in 



The cohesive property of water was determined as long 

 ago as 1846 by Donny, but till the appearance of Dixon 

 and Joly's researches it was thought the water must be free 

 from air. These writers proved that this is not essential. 



They determined that under the conditions in which the 

 columns of water exist in the wood they will cohere under 

 a tension of about seven atmospheres, which is sufficient 

 to raise the liquid as high as the leaves at the summit of 

 a tall tree. 



The forces necessary to exert this traction, naturally 

 sought for in the leaves, were described differently by the 

 Irish and German observers. The former suggested ' sur- 

 face tension forces, developed in the substance of the walls 

 of the evaporating cells '. Askenasy spoke of imbibition 

 by the cell walls. Both authors claim, however, that 

 a second force can be found in the osmotic suction of the 

 mesophyll cells, which draw water from the tops of the 

 columns as the turgescence of the leaf tends to fall, and 

 they agree further in saying it is probably the more power- 

 ful of the two. The relation between the two is put forward 

 very clearly by Askenasy. 



A theory so startling to botanists as this one naturally 

 became the subject of much controversy, but was not re- 

 garded up to the end of the century as a satisfactory 

 explanation of the phenomena. No complete agreement 

 was arrived at as to the resistance to the flow of the 

 transpiration current, which was very variously estimated. 

 Schwendener computed that the suction power of the leaves 

 is altogether inadequate to exert the necessary force in 

 fact he held it to be incapable of extending further back- 

 wards than 5 inches. Both he and Steinbrink also dis- 

 puted the ascent of the entire Jamin's chain as a coherent 

 column, basing their objections on the alterations of tension 

 in the air-bubbles during the traction. The movement of 

 the whole column is in antagonism to some observations by 



