HAIRS OX LEAVES 119 



nearly vertical, the stomata are distributed somewhat equally 

 on both surfaces. Stomata occur in the epidermis of young 

 stems, being replaced later by the lenticels. 



The health of the plant depends largely on the proper work- 

 ing condition of the stomata, and one reason why plants in cities 

 often fail to thrive is that the stomata become choked with dust 

 and soot. If the stomata were to become filled with water, their 

 activity would cease until they were freed from it ; hence many 

 plants have their leaves, especially the under surface, protected 

 by a coating of wax which sheds water. 



143. Hairs on leaves. Many kinds of leaves are more or less 

 hairy or downy, as those of the mullein, the " mullein pink," 

 many cinquefoils, and other common plants. In some instances 

 this hairiness may be a protection against snails or other small 

 leaf-eating animals, but in other cases it seems to be pretty clear 

 that the woolliness (so often confined to the under surface) is 

 to lessen the loss of water through the stomata. The Labrador 

 tea is an excellent example of a plant with a densely woolly 

 coating on the lower surface of the leaf. The leaves, too, are 

 partly rolled up like those of the crowberry (Fig. 361), but less 

 completely, with the upper surface outward, so as to give the 

 lower surface a sort of deeply grooved form, and on the lower 

 surface all of the stomata are placed. This plant, like some 

 others with the same characteristics, ranges far north into regions 

 where the temperature, even during summer, often falls so low 

 that absorption of water by the roots ceases, since it has been 

 shown that this nearly stops a little above the freezing point of 

 water. Exposed to cold, dry winds, the plant would then often 

 be killed by complete drying if it were not for the protection 

 afforded by the woolly, channeled under surfaces of the leaves. 



144. Total amount of transpiration. In order to prevent 

 wilting, the rise of sap during the life of the leaf must have 

 kept pace with the evaporation from its surface. The total 

 amount of water that travels through the roots, stems, and 

 leaves of most seed plants during their lifetime is large, relative 



