About Green Leaves 



199 



chokes the little mouths, transpiration is stopped, 

 and the tree's work comes to a standstill. 



Hence, most leaves are so made that they can 

 shed water like new umbrellas. Sometimes the 

 leaf-skin is so smooth that water runs off it — as 

 it does from polished tiling. Sometimes it is 

 waxed, so that it can shed the rain drops, and 

 often it contains a substance called " cutin," which 

 makes it waterproof. 



On many trees, rain drips from leaf-point to 

 leaf-point, and is 



Fig. SI. Part of the upper surface of a 

 leaf, showing one of the stomata with its 

 lips closed (very much magnified). 



shed as it would 

 be from the shin- 

 gles of a roof. 

 On many low- 

 growing plants it 

 runs down the 

 leaves, toward 

 the main stem, and so reaches the roots. 



All through a bright summer day leaves and 

 green twigs are breathing out vapor into the air. 

 The rate of this slow steaming of the forest 

 depends upon many things — the season, the 

 weather, the temperature, and the nature of the 

 trees. 



It is easy to test in a simple way this evapora- 

 tion of summer boughs. 



Cut a growing, leaf-covered twig from some 

 broad-leafed tree — such as a maple or cotton- 

 wood. " Put the cut end of the twig through a 



