TRANSPIRATION AND ASCENT OF SAP DIXON. 



425 



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 mini- 

 mal resistance of the porous and thin mem- 

 brane. But the very delicacy and porosity of 

 the membrane render it unsuitable for sustain- 

 ing any severe stress, and so 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 asjnrated away from the 

 bubble, and the membrane is supported by the 

 dome, while the torus lies over the perforation 

 in the latter like a washer or plug. (See fig. 4.) 

 In this position of the membrane the tension 

 of the water and the gas pressure are with- 

 stood, 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. 



ThiiSj 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, by an automatic change; a rigid support to the tensile 

 sap and oppose an impermeable barrier to undissolved gas. * * * 



