214 A YEAR IN SCIENCE 



The outer portion of the cell walls contains a cork-like 

 substance which is waterproof. The surface of the 

 leaves of some plants is covered by a film known as 

 the bloom. If this is removed, evaporation takes place 

 much more rapidly. You have probably noticed this 

 bloom on some leaves such as the tulip, and certainly 

 on fruits like grapes and plums. Sometimes the leaves 

 are covered with soft hairs. In some instances, as the 

 mullein, these hairs are very numerous. They are also 

 numerous on very young leaves. This is especially 

 evident on young fern leaves. These hairs are known 

 to retard evaporation. 



Stomates serve to admit air to the interior of the 

 leaf, and they also allow moisture in the form of vapor 

 to pass out. By changes in the guard cells, the stomates 

 of many plants may be opened and closed. When they 

 are open, the gas exchanges necessary for food-making 

 are facilitated. This condition, however, also increases 

 transpiration. The presence of stomates only on the 

 under surface of most leaves is probably an advantage 

 in reducing transpiration. On leaves which stand erect, 

 as grasses, they are almost equally distributed on both 

 sides, and on those which lie on the surface of the 

 water, as the giant water lily, they are present only 

 on the upper surface, 



Many plants shed their leaves at the beginning of a 

 cold or a dry period. This is, no doubt, of great 

 advantage to the plant because it results in an enormous 

 reduction of transpiration. Plants which have the 



