214 



LEAVES 



The possession of hairs may have some advantages to 

 the plant which are not understood ; one advantage that 

 has been proved, however, is that hairs when abundant 

 retard evaporation. It is significant that hairs are usually 

 more abundant on the under surfaces of leaves, and that 

 it is on the under surfaces that the majority of stomates are 

 usually located. Some leaves are densely hairy below 



and perfectly smooth 

 above. Tender, 

 young leaves often 

 have hairs which dis- 

 appear when these 

 leaves have become 

 older and their sur- 

 faces have become 

 more waterproof. 



It is not surprising 

 to find that hairiness 

 and thickening of the 

 epidermal walls by 

 cutin are quite gen- 

 eral among alpine and 

 northern plants, for' 

 the danger of death by loss of water is even greater in 

 cold regions than in warm regions. In cold regions the 

 soil is often much colder than the air, and this makes it 

 more difficult for absorption to keep pace with evapora- 

 tion. By alpine plants we mean those that grow high on 

 the mountains. The heath family is a large family of 

 plants which is more abundant in cold and alpine regions 

 than elsewhere, and the leaves of these plants are nearly 

 always either hairy or coated with wax or cutinized. Bear 



Fig. 77. 



- Microscopic view of the hairs of a 

 leaf of mullein. 



