THE FUNCTIONS OF THE WOOD 203 



It must be remembered that the inflow of water from 

 the ground to the elements of the wood of the roots takes 

 place across the cortical cells of the root. For, though 

 the latter have a much higher osmotic pressure than have 

 the tracheae, they function merely as a complex semi 

 permeable membrane, as they are already fully distended. 

 Leclerc du Sablon (1911), in the absence of measurements 

 of the osmotic pressure of the tracheae, imagined it to be 

 greater than that of the cortical cells. Consequently he 

 explains root pressure as a flow occasioned by the osmotic 

 gradient, from ground, through cortex to tracheae. By 

 implication this view requires the cortical cells to secrete 

 a more concentrated sugar solution than they contain, 

 even though pure water is available continually to dilute 

 it. The facts experimentally ascertained, however, show 

 that such a secretion does not take place, for the sugar in 

 the tracheae is far less concentrated than in the cells. 

 Thus the view put forward here differs considerably from 

 that of Leclerc du Sablon. 



Where the wood parenchyma is specially concentrated 

 round the vessels, while the tracheids form comparatively 

 large tracts without living cells intermingled, we may with 

 probability assume that a certain amount of division of 

 labour has come about, and that the vessels function as 

 the principal channels for the transmission of carbo- 

 hydrates. The tracheids, on the other hand, are chiefly 

 concerned with the upward conveyance of the water. Of 

 course, as long as no bubbles are developed in the vessels 

 they will transmit the sap in them, with all its constituents, 

 more rapidly than will the tracheids, when both are under 

 the tension generated by the leaves. But if a bubble arises 

 the stream will be deflected into the tracheidal columns. 

 There, owing to the smaller size of the compartments com- 

 posing the channels, the tensile column will acquire greater 



