50 Henry H. Dixon. 



a washer or plug (see Fig-. 5). In this position of the membrane 

 the tension of the water and the gas pressure are withstood, not 

 by the thin and delicate membrane, but by the surface of the 

 water supported by the denser and more rigid material of the wall 

 and of the torus, while the delicate membrane is shielded from all 

 stress. 



Thus from the standpoint of the tension hypothesis we regard 

 the bordered pits as mechanisms to render the walls as permeable 

 as possible to continuous water streams, while, when conditions require 

 they provide, hj an automatic change, a rigid support to the tensile 

 sap and oppose an impermeable barrier to undissolved gas. 



In the foregoing we have seen that, when water is abstracted 

 from the leaves of plants by evaporation, the properties of water and 

 the conditions in the conducting tracts necessitate that the contained 

 water shall pass into a state of tension. This is always true unless 

 root-pressure or atmospheric pressure or the combined action of 

 both at the time is forcing up water more rapidly than evaporation 

 eliminates it. The structure of the wood makes the fluid stable 

 under the tension and at the same time combines the advantages of 

 very considerable permeability with the great rigidity needed to 

 transmit the stress. 



When root-pressure is not vigorous the tensile stress of the sap 

 must be transmitted across the parenchymatous cells of the root 

 terminations to the surfaces of the root hairs. In these cells the 

 actions which occur must be the converse of those occurring in the 

 mesophyll. At the root the entry of water depends on the gradient 

 of pressure as we pass from the outside of the root to the inside of 

 the tracheae. The fall of pressure due to the tension in the water 

 is continuous all the way up the stem to the leaf Thus we may 

 regard the flow of water up the highest tree as due to the evaporation 

 and condensation produced by the difference between the vapour 

 pressure in the soil spaces and that obtaining round the leaves. The 

 column of tensile water flows under the action of this difference from 

 end to end of the plant. 



The relations in these respects of the leaves to the roots may 

 be illustrated \) by two porous pots connected hermetically by a glass 

 tube about a metre long, the pots and the tube being completely 

 filled with water (Fig. 6j. If one porous pot is immersed in damp 

 earth and the other supported above it, it will be found that the 

 difference in the state of saturation of the spaces surrounding each 



1) H. H. Dixon, On the Physics of the Transpiration Current. Notes from 

 the Botanical School, Trinity College, Dublin, No. 2, 1897, p. 20. 



