THE ASCENT OF SAP 267 



hesion theory untenable, is a question which has been much de- 

 bated, but it seems that because of the irregularities in the shape 

 of the cell wall, caused by the spiral thickenings and other mark- 

 ings, the air bubbles never completely fill the cell and thus break 

 the columns of water. Or if such a rupture does happen, then the 

 water is continuous through the cell wall and there is still an un- 

 broken strand of water. According to this conception, therefore, 

 whole sections of the wood might be filled with air without break- 

 ing the water rope as a whole. That is, the transpiration stream 

 is not to be considered as a group of separated water columns but 

 as a unit which maintains its unity and integrity throughout the 

 life of the plant. Votchal (1897) even conceived that the air bub- 

 bles acted as springs which, by providing increased resiliency, 

 aided rather than impaired the efficiency of the column. Mac- 

 Dougal, Overton, and Smith (1929) concluded that varying gas 

 pressures inside the tree had little effect on the cohesive water 

 system, since changes in gas pressure inside the tree registered on 

 attached gas manometers but did not register on water manom- 

 eters, indicating that the internal gas system has little or no con- 

 nection with the water system. 



Other objections have been made to the cross walls of the ves- 

 sels. While these walls are sometimes a meter or more apart, 

 the average length of a vessel is probably less than ten centimeters; 

 and it has been argued that the water could hardly be pulled 

 through these cross walls in the fashion demanded by the hypothe- 

 sis. Since these walls are full of imbibed water, the molecules of 

 which are in constant touch with one another, they would not seem 

 to offer any insurmountable difficulty to the cohesion theory of the 

 ascent of sap. At all events this theory, although confronted with 

 many facts to explain, has made much progress. It offers a con- 

 cise, physical explanation of phenomena which were long thought 

 to be mysterious and unexplainable; and while root pressure, 

 capillarity, and atmospheric pressure may play minor roles in 

 lifting the transpiration stream, the most potent force is the 

 evaporative pull of transpiration, which the cohesion of water 

 permits to be transmitted to the bottom of the plant. 



