90 FLACCID SHOOT. [CH. IV 



to understand the following experiment 1 . A piece of a 

 yew (Taxus) branch (4 or 5 cm. in length) is placed in water 

 until thoroughly soaked. When removed from the water 

 and held with its axis vertical no water escapes from the 

 lower surface (although water is contained in the tracheids) 

 because if water is to escape air must enter, and air does 

 not easily pass wet membranes. If however a drop of 

 water is added (e.g. with a wet paint-brush) to the upper 

 surface, water immediately oozes out below. 



(103) Permeability of splint wood. 



A modification of the experiment may be used to 

 illustrate the fact that the water travels in the splint- 

 wood. A piece of yew branch (5 or 6 cm.) is fitted to a 

 rubber tube of 50 or 60 cm. in length. The tube is filled 

 with water and closed below 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 2 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. 



1 See Sachs in his Arleiten, n. p. 296. Also Godlewski, Primjsheim's 

 Jahrbttcher, xv. 



2 Sachs' Arleiten, i. p. 287. 



