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 filled with water. Their form and the thickness and 

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



ch 



FIG. 113. Surf ace *view of the epidermis 

 of a buttercup leaf l 



e, cells of epidermis ; n, nuclei of epidermal 

 cells ; g, 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 R. acris. 



