THE LEAF. 69 



nary foliage leaf shows that both surfaces are protected by 

 an external layer of cells constituting the epi- , ^ . , 



. •' or Anatomical 



dermis. The outer wall of the epidermal cells structure, 

 is commonly thickened, and by taking on a ^^ "™^' 

 layer of cutin or wax becomes nearly or quite impervious 

 to water. The leaves of some plants, particularly of species 

 growing in tropical regions, have more than one layer of 

 cells composing the epidermis, thus securing more efficient 

 protection. The cells of the epidermis are, for the most 

 part, destitute of chlorophyll, but contain a large quantity 

 of water which is absorbed as required by the delicate cells 

 in the interior of the leaf. Additional protection is often 

 afforded by hairs which thickly cover the leaves of many 

 species, particularly those growing on the steppes and 

 other parts of the globe where vegetation is subject to 

 sudden and extreme changes of temperature. Finally, 

 protection is not infrequently secured by diminishing the 

 amount of leaf surface, as seen in many shrubs, and in 

 desert grasses and sedges with cylindrical leaves. 



Communication with the interior of the leaf is secured 

 by means of numerous openings called stomata. These 

 are provided with guard-cells, commonly of the 

 same general form as those of the hyacinth leaf, 

 which act as a valve, opening in sunlight while the leaf is 

 at work and closing, or partially closing, at night. The 

 mechanism, apparently simple, is, in reality, rather diffi- 

 cult of complete explanation.^ The essential fact is that 

 by means of the stomata a free interchange of watery 

 vapor and gases between the interior of the leaf and the 

 surrounding atmosphere is effected, and that by means 

 of the guard-cells this interchange is obstructed when the 

 external conditions are unfavorable. 



1 Cf. Sachs, Physiology of Plants, pp. 248-251. 



