480 Forestry Quarterly 



Dixon's theory of the ascent of sap, elaborated in great detail 

 in the present volume, assttmes that the water in the conducting 

 tubes of high trees hangs by virtue of its cohesion, re-enforced by 

 its adhesion to the walls of the conduits. These surface tension 

 forces are greater than that of gravity, so the water hangs sus- 

 pended in the minute conducting tubes. When columns of water 

 adhere completely to a rigid envelope, as is the case in the con- 

 ducting tubes, it assumes a pseudo-rigidity, and it is capable of 

 sustaining and transmitting tensile stresses. Experiments with 

 tension tubes showed that air-free water can sustain a tension 

 whose minor limit was equivalent to 50 atmospheres pressure. 

 The author's experiments with cell sap from ilex and beech dem- 

 onstrated that, under various conditions of temperature, it could 

 withstand a tension varying from 47 to 207 atmospheres. The 

 experiments were necessarily made with glass tubes. It was found, 

 however, when pieces of wood were introduced into the tubes, that 

 the rupture would always take place along the surface of the glass, 

 not along the wood surface, thus indicating that water in wooden 

 tubes, as in the plant, would stand a greater stress than experi- 

 mentally determined in glass tubes. Resistance to a current of 

 water moving through wood at the velocity of the transpiration 

 stream is approximately equivalent to a head of water equal in 

 length to the wood traversed. Hence the tension applied to the 

 upper end of the water columns, which will be able to raise the 

 transpiration stream in a tree must be equal to the pressure pro- 

 duced by a head of water twice the height of the tree. In a tree 

 100 meters high, therefore, a tension of 20 atmospheres must be 

 produced. 



If the evaporation from the outer surfaces of the mesophyll 

 cell of the leaves is extracting water from the conducting tubes 

 more rapidly than the lifting forces can supply it, then the water 

 in the tubes must fall into a state of tension. The only effective 

 lifting forces are those of atmospheric pressure and root pressure. 

 The former cannot supply water higher than 33 feet, and the 

 amount suppUed by the latter is insignificant compared with the 

 losses due to evaporation. The water in the conducting tubes 

 above this level is always in a condition of tension, and in times 

 of vigorous transpiration, whenever the loss cannot be made good 

 by the lifting pressure of the atmosphere, the water in the con- 

 ducting tubes below 33 feet is doubtless also in a state of tension. 



