80 NEGATIVE PRESSURE. [CH. IV 



again, repeat the removal of a shaving of wood to prove 

 that this per se has no special effect. 



(99) Negative pressure. 



A similar result may be got more quickly by filling the 

 potometer with an emulsion of skim-milk diluted with three 

 times its volume of water. The slowing of the current, 

 and the recovery when the blocked ends of the vessels are 

 cut off, may thus be seen within a short space of time. 



(100) Negative pressure. 



The fact of the existence of negative pressure may 

 best be demonstrated by von Hohnel's method'. If a 

 strongly transpiring stem or branch be cut under a watery 

 solution of eosin, immediately removed and examined, 

 the red colour will be found to have rushed into the 

 vessels to a considerable height. We find that " Solomon's 

 seal" {Polygmiatum multiflorum) answers well. Or plants 

 of Helianthus tuberosus grown in pots and left without 

 water until they are on the point of withering. In winter, 

 seedlings of Vicia faha pulled out of the loose sawdust in 

 which they have grown, and allowed to lie on the table 

 until nearly withered, show good injection. 



(101) Permeability of cell membranes. 



Negative pressure depends on the fact that dry cell 

 membranes are far more easily permeable to air than are 

 wet membranes, (i) Take a cylinder 2 inches long by 

 \ inch diameter, turned from the splint wood of a fresh 

 branch of Yew or of Pinus, and attach it by a strong 

 ^ Pringshevm's Jahrbucher, xii. 1879, p. 47. 



