204 SECTIONAL ADDRESSES 



experiment we have failed to displace the liquid contents of closed hard- 

 wood vessels by air, using pressures of 15 atmospheres. 



The displacement of water by air entering through these pores would 

 not in any case be easy, and when it is remembered how they arise it 

 will be seen to be practically impossible. The pores were filled originally 

 by protoplasmic connections. These are found only connecting adjacent 

 protoplasts, and only persist as pores in thin-walled pit areas where the 

 original wood elements have not been displaced relatively during 

 development. 



The pores, then, do not open on to intercellular spaces and are not in 

 communication with the air of the intercellular system unless considerable 

 splitting apart of the primary walls of adjacent tracheal elements has 

 occurred ; such splitting only occurs, if at all, in dry heart wood in which 

 the water movements are relatively unimportant. We may conclude then 

 that the liquid contents of the tracheal elements may be under tension 

 without the slightest likelihood of air being drawn in from the intercellular 

 system of the wood to displace the liquid. 



Water.— Ent the tension upon the water contents of the tracheal 

 elements must increase very rapidly if the water cannot be displaced by 

 gas. When the water in a trachea is in tension, at points where the wall 

 faces upon an air space the water will be withdrawn into the wall until 

 the menisci are very concave and only a thin film of water, no longer free 

 to move, covers the internal (inter-micellar) surfaces. This is not only 

 true of the wall where it faces upon an air space, but the water in all the 

 wall must be in equilibrium with this and will be withdrawn into the 

 trachea until only a thin film is left. In the cohesion theory of the ascent 

 of sap the high tension which must develop in the contents, if the tracheae 

 remain full of sap, is held accountable for the upward movement of 

 water in the tree. But this tension must isolate the liquid contents of 

 the trachea, because the walls between this trachea and its neighbours 

 will only have a water content in equilibrium with the walls bordering 

 air spaces, and the movement of water across such walls, under the differ- 

 ence of tension developed between the tracheal contents, will be very 

 slow indeed. Indeed, as Schwendener and Nageli pointed out many 

 years ago, when tensions develop the liquid contents of the tracheas are 

 immobilised, with the result that the rate of evaporation from the mesophyll 

 is cut down. 



Water Vapour. — Nevertheless, though the rate of evaporation falls 

 on a sunny day with the water in the tracheae immobilised, if they all 

 remain full of water the tensions developing in the tracheae near the 

 leaves would be very high indeed. 



Experimental evidence however, such as was supplied originally by 

 von Hohnel and Scheit, suggests that water vapour replaces water in many 

 tracheal elements. As soon as a bubble of water vapour forms in any 

 trachea, its water will be immediately free to move and dispersed into the 

 surrounding tracheae in which tension will be released, and thus water 

 remains available for the tracheae supplying the evaporating leaf without 

 a great rise in the tension of their contents. 



The presence of tracheal elements containing water vapour may be 



