LEAVES 



193 



like opening. These cells are called guard-cells, because they 

 regulate the size of the opening. When seen in the cross- 

 section of the epidermis, the stomata appear as pairs of small 

 cells interrupting the epidermal layer (Fig. 157). The guard- 

 cells usually project toward one another near the centre of 

 their depth, so that the opening between them funnels down 

 to a narrow slit and then enlarges, on the general plan of 

 an hour-glass. The guard-cells can change their shape, 

 and so enlarge or diminish the opening between them. It 



FIG. 159. Surface view of epidermis of a hyacinth leaf: A, elongated epidermal 

 cells and four stomata with their guard-cells ; B, enlarged view of a single stoma. 



is the stomata that provide passageways into the interior 

 of the leaf, solving the problem of the epidermis how to 

 prevent excessive loss of water and at the same time main- 

 tain communication with the air. 



The number of stomata is a very important feature of the 

 mechanism of a leaf, for it is found that very numerous small 

 openings over a given area are as effective in gas exchanges 

 as if the whole area were open. A fair average number of 

 stomata is about 100 to each square millimeter of surface 

 (about 62,500 to the square inch) ; and in some cases the 

 number may reach 700 to the square millimeter (almost 

 450,000 to the square inch). When it is remembered that 



