104 



STRUCTURE AND FUNCTIONS OF LEAVES 



-work of the palisade cells, besides evaporating much water. The 

 stomata admit air to the interior of the leaf, where the air spaces 

 serve to store and to distribute it ; they allow oxygen and carbon 

 dioxide gas to escape ; and, above all, they regulate the evapora- 

 tion of water from the plant. 



123. The epidermis. The cells of the epidermis are very gen- 

 erally rilled with water. Their form and the thickness and 

 material of their walls depend largely on the kind of soil and 



FIG. 113. Surface view of the epidermis 

 of a buttercup leaf l 



e, cells of epidermis ; n, nuclei of epidermal 

 cells ; #, guard cell of stoma ; s, stoma. 

 Much magnified. After Giesenhagen 



FIG. 114. Section through 

 stoma of a buttercup 

 leaf, at right angles to 

 epidermis 



e, epidermal cells; g, guard 

 cell of stoma ; s, stoma ; 

 ch, air chamber. Much 

 magnified. After Bonnier 

 and Sablon 



climate to which the plant ^is adapted. In most herbs the epi- 

 dermal cells form only a single layer and are not greatly 

 thickened. 



The stomata are not mere holes in the epidermis, but have a 

 somewhat complicated structure. Each stoma consists of two 

 kidney-shaped guard cells inclosing a slit-like opening into the 

 leaf (Fig. 113). 



When the stoma is viewed in a section at right angles to the 

 surface of the leaf (Fig. 114) it appears as a narrow passage 

 communicating with an air chamber inside the epidermis. 



The number of stomata in a square inch of leaf surface is 

 very great. An apple leaf contains about 24,000 and a black 



1 Fig. 113 is from Ranunculus Ficaria ; Figs. 114-118 from E. acris. 



