82 FLACCID SHOOT. [CH. IV 



wood. A piece of yew-branch (2 — 3 iuches) is fitted to a 

 rubber tube of about 3 feet in length. The tube is now 

 filled with water and closed by a clip. If the wood is 

 held vertically with tube hanging straight down, the upper 

 surface of the wood, which must be cut smooth, is dry. 

 If the closed end of the tube is raised until it is slightly 

 above the top of the branch, the surface of the young 

 wood is seen to blush or change colour, even before the 

 water can be seen to actually ooze from it. 



(104) Recovery of a flaccid shoot. 



De Vries has shown ^ that when a shoot is cut in the 

 air it frequently withers after it has been placed in water. 

 This has usually been explained as being due to the air 

 rushing in under negative pressure and filling the vessels. 

 It is not quite clear that this is the cause, but whatever 

 the explanation may be, it is interesting to note that a 

 shoot which has been rendered flaccid, by being cut in the 

 air and allowed to partially wither, can be rapidly restored 

 to turgescence by forcing water into its vessels under 

 pressure, — which takes the place of the suction of nega- 

 tive pressure. The cut end of such a withered shoot is 

 attached by a rubber tube to the short arm of a U tube 

 containing water. The position of the end of the shoot, 

 which droops flaccidly over, is noted on a vertical scale, 

 and then mercury is poured into the long arm. Under 

 the pressure of about 10 cm. mercury, the plant recovers 

 and the end of the shoot can be traced with the naked 

 eye rapidly travelling up the scale. 



1 Sachs' Arheiten, i. p. 287. 



