xi ABSORPTION AND MOVEMENT OF WATER 143 



in the guard cells returns to its normal strength. Figures 157 

 and 158 show the opening and closing of the stomata. 



Lenticels and Transpiration. The lenticels (p. 108) 

 which are formed in the periderm of a woody plant also give 

 out water vapour ; but the quantity so lost can only be small. 

 The lenticels communicate with the intercellular spaces in the 

 plant, much in the same way as stomata communicate with the 

 intercellular spaces in the leaf. In winter the lenticels are 

 closed by ordinary periderm, but they are open in summer. 



Force exerted by Transpiring Shoots. If a branch is 

 cut from a tree, and the cut end is placed in water, it will re- 



FIG. 158. Stoma in transverse section. The darker lines show the shape of the 

 stoma when open, and the lighter lines when closed. (S.) 



main fresh. This shows that the branch can take in water by 

 its cut end. The force which such a branch can exert while 

 actively transpiring can be measured by the following ex- 

 periment. 



EXPT. 134. Cut a branch from an Oak tree when the leaves are 

 fully developed, and fix it in an air-tight manner in a glass tube filled 

 with water, the lower end of which dips into a cup of mercury. Note 



(i} The volume of the water decreases. 



(?.i) The mercury rises in the tube. 



(iii) This is caused by the suction exerted by the transpiring shoot. 



(iv) The water in the tube disappears to make good that lost by 

 transpiration. 



(v) Thus the mercury is forced up the tube by atmospheric pressure. 



Conditions Necessary for Transpiration. 

 i. A certain intensity of light ; the stronger the light the 

 greater the transpiration. 



