84 THE NATURE AND WOBK OF PLANTS 



105. Regulation and control of transpiration. — 

 The leaf is able to control the amount of water 

 given off by opening and closing the stomata. The 

 amount of moisture already in the air influences 

 the transpiration greatly, and as this amount varies 

 widely in different places there are a large number 

 of forms of leaves adapted to the different conditions. 

 The leaf may shield itself from the drying effects of 

 intense sunlight by heavy cuticles, coats of hair, or 

 wax, or by the upright positions described in § 93. 



106. Wax or bloom as a means of prevention of 

 excessive loss of water. — Select some leaf which has 

 the surfaces covered with a whitish bloom (cabbage). 

 Rub the bloom from one and place it by the side of a 

 second which has been handled carefully in order not 

 to disturb the bloom. Which is the more wilted in 

 two or three hours ? The best results will be obtained 

 if the leaves are placed in the sunlight. 



107. Size of leaves and dryness of the air. — Spe- 

 cies growing in dry air generally exhibit very small 

 leaves, while those living in wet places or where the 

 air is very damp develop large laminae. It is neces- 

 sary to have a stream of water constantly travelling 

 from the roots to the leaves, and plants living in 



