THE STOMATA 



125 



FIG. 62. A typical stoma in cross section 

 and surface view combined, k, guard cell; ;', 

 the gap or stoma between the guard cells; /, 

 epidermal cell bordering a guard cell. 



The Stomata. The stomata are minute openings, invisible 

 to the naked eye, through the epidermis. They may be found 

 on any part of plants except 

 the roots, and they occur in 

 greatest numbers in the 

 leaves, where they average 

 100 to 300 per square milli- 

 meter; and sometimes, as in 

 species of Olea and Brassica, 

 they become as numerous as 

 600 to 700 per square milli- 

 meter. 



A stoma is guarded by two 

 guard cells (Fig. 62), which, 

 as a rule, have the power automatically to open and close the 

 stoma. The formation of a stoma comes about by the division 



of a protodermal mother cell 

 into two daughter cells and the 

 dissolution of the middle 

 lamella of the wall separating 

 these, thus forming a crevice, 

 which may be closed, or opened 

 out by the action of the guard 

 cells to a breadth of approx- 

 imately .008 mm., or about 

 one-nineteenth the thickness of 

 this page (Fig. 63). So minute 

 are these clefts in the epi- 

 dermis, even in their wide- 

 open condition, that their value 

 as entrance ways for carbon 

 dioxide gas cannot be appre- 

 ciated without experimental 

 data. This, fortunately, is at 

 hand. It has been found in purely physical experiments that 

 this gas will diffuse at a faster rate through very minute open- 



FIG. 63. A, diagram showing relative 

 position of the guard cells in cross section in 

 the open and closed positions; the heavier 

 line indicates the open position. B and C, 

 early stages in the formation of stomata; 

 at 5, mother cells of guard cells are shown. 

 D, s, s, two guard cells formed by the 

 division of a mother cell. (After Sachs.) 



