iv COHESION THEORY OF ASCENT OF SAP 



99 



velocities which Pappenheim found were necessary to 

 deflect the membrane, and, of course, hydrostatic tension 

 in the liquid on each side of the membrane will not tend 

 to displace it. Hence it is that the tensile transpiration 

 current, passing from one trachea to another through the 

 bordered pits, experiences only the very small resistance of 

 the porous and thin membrane. But the very delicacy 

 and porosity of the membrane render it unsuitable for 

 sustaining any severe stress. Hence we find, when a bubble 

 develops in a trachea and is gradually distended by the 

 tension in the liquid, or by a difference of gas pressure, 

 till it fills the trachea, the membranes 

 of the pits in the walls of the trachea 

 become aspirated away from the 

 bubble, and the membrane is sup- 

 ported by the dome, while the torus 

 lies over the perforation in the latter 

 like a valve on its seat (.see Fig. 18). 

 In this position of the membrane the 

 tension of the water and the gas pres- 

 sure 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 point of view of the 

 tension hypothesis, we regard the bor- 

 dered pits as mechanisms to render the 

 walls as permeable as possible to con- 

 tinuous water streams, while, when conditions require, 

 they provide, by an automatic change, a rigid support 

 to the tensile sap and oppose an impermeable barrier to 

 undissolved gas. 



Fig. 18. 



H 2 



