Current Literature 481 



Since the wall of the tubes is permeable to water, the water in 

 one conducting tube is continuous with that in its neighbors, and 

 consequently the tension in one is transmitted to the water in the 

 adjacent conduits. Thus, when the leaves of a tree are trans- 

 spiring, the cohesion of their sap explains fully the transmission of 

 the tension downwards and consequently explains the rise of the 

 sap. 



When evaporation from the transpiring cells removes water 

 faster than it can be supplied, the menisci formed in the substance 

 of their walls support the tensile colimms of water in the plant. 

 Evaporation from these menisci provides the traction to raise the 

 water. The entry of water at the root depends upon the gradient 

 of pressure on passing from the outside of the root to the inside 

 of the conducting tubes. The fall of pressure due to the tension 

 of water is continuous all up the stem to the leaf. The flow of 

 water up the highest tree is due, then, to the evaporation and condensa- 

 tion produced by the difference between the vapor pressure in the soil 

 spaces and that obtaining around the leaves. 



The ultimate source of energy for evaporation from the meso- 

 phyll cells of the leaf is heat. Evaporation lowers the temperature 

 because the less energetic molecules are left behind in the process. 

 This causes the heat from the surroundings to flow inwards, which 

 in turn stimulate the activity of the molecules, and so continues 

 the process of evaporation. Thus an evaporating leaf acts as 

 "a sink of energy." The transpiring mesophyll cells of the leaf 

 nominally remain turgid during transpiration. Thus, in tall 

 trees, the osmotic pressure keeping the cells distended must 

 correspond in magnitude to the tensions necessary to raise the sap. 

 The author found these pressures always sufficient to resist the 

 transpiration tension. The author believes that water lost by 

 evaporation through the stomata may be secreted on the outside 

 walls of the cells lining the stomatal chamber by the vital activity 

 of the protoplasm. By various calculations, he shows that the 

 stored energy set free by respiration is quite sufficient to do the 

 work of secretion against the resistance of the transpiration stream. 



It will be evident upon consideration of the above outline of 

 Dixon's theory of the ascent of sap, that while he does not consider 

 the living cells in the stems as active agents in the process, the 

 activity of living cells in the leaves is quite necessary to uphold 

 his theory. The author believes that the structure of the various 



