Dixon — The Cohesion Theory of the Ascent of Sap. 53 



in the tube, will set up local differences of pressure, which the 

 impermeability of the plaster will maintain for long periods. 



1. Copeland states : " The tension of the water in the plaster 

 and that of the air and water around it at the same height must 

 be practically the same." So far as this statement refers to the 

 practical equality of pressure in " air " and water, it is certainly 

 mistaken. The pressure conditions in free gas contained in water 

 involve, in all cases, as one factor the surface-tension of the 

 meniscus enclosing the gas, and are necessarily positive from the 

 nature of gases. These conditions are consequently not continuous 

 with the pressure-conditions of the containing liquid, which may 

 be either positive or negative. Thus, although we might infer 

 the state of tension of a liquid by observing the maximum 

 diameter of bubbles contained in it (and whose surface-tension is, 

 therefore, necessarily in equilibrium with the stress in that region 

 of the fluid), the gas within the bubbles is actually under positive 

 pressure-conditions imposed upon it by this very surface-tension. 



Manometers, then, which would register the pressure-conditions 

 obtaining simultaneously in the bubbles and in the water of 

 Copeland's tube, would give very different readings. 



If the bubbles at any level are sufficiently restrained by the 

 plaster, the gravitational pull acting against the surface-tension 

 forces developed in the piaster will slowly put the water into a 

 state of tension ; while, of course, the gas in the bubbles would at 

 the same time be in a state of positive pressure. 



But we have no reason to believe that the manometers in the 

 experiment were in a condition to indicate tension in the liquid, 

 even if such existed. For Copeland does not state he took any 

 special precautions to secure unbroken continuity between the 

 water in the plaster of Paris and the mercury of his manometers. 1 



1 The neglect of securing proper continuity in the water experimented on is also 

 probably responsible for the results quoted by Copeland to show that branches cannot 

 take up water, unless tbe latter is supplied under pressure. In these cases, as soon as 

 the pressure is much reduced, discontinuities appear in the water which has been raised 

 into capillaries opening on the cut surface. As transpiration proceeds, these ruptures 

 will enlarge till they become bubbles occupying the whole of the open capillaries. 

 Nothing comparable to this, of course, occurs in the living tree, where, as a matter of 

 fact, we know that water passes up in presence of gas-pressure amounting to less than, 

 one atmosphere. 



