562 



ECOLOGY 



have no stomata, whereas emersed leaves have them in abundance (figs. 

 768, 769). Subterranean scale leaves usually have no stomata, yet 

 when the primordia of such leaves are exposed to light and air, they 

 often develop into foliage leaves with abundant stomata, while the 

 primordia of foliage leaves when grown in the soil become scale leaves 

 and are devoid of stomata. Sometimes stomatal development begins 

 in the soil, but it is completed only in the light and air. 



The factors involved. — The factors that induce the appearance of stomata are 

 not known, though in most instances air is a necessary medium for their development. 

 It has not been ascertained which element of the air is the most important, nor what 

 is the nature of its influence. There is some evidence also that light tends to favor 

 the development of stomata. Since stomatal activity is confined to the air, it is 

 possible that there is some relation between gas exchange and stomatal development. 



Fig. 806. — • A diagrammatic cross section through a stoma of Helleborus, illustrat- 

 ing guard-cell movement ; the solid lines represent the open position and the dotted lines 

 the closed position; closure involves the movement toward the central slit (5) of the 

 ventral wall (f) and the dorsal wall (d), these walls assuming the positions, t' and d', re- 

 spectively; the hinge ih) moves in the same direction, assuming the position, h'\ the 

 outer walls {w) remain immobile; the movements of the ventral and the dorsal walls 

 decrease the sphericity of the cell lumen (J); r', inner ridges; general lettering as in fig. 

 795 ; highly magnified. — From Schwendenee. 



In particular, stomata are abundant where transpiration is vigorous, and absent 

 where it is reduced or wanting. 



The mechanism of stoma movement. — Guard-cell movements are extremely 

 complex and not clearly understood, while their amount and importance often are 

 overestimated. They are best illustrated in mesophytes, the stomata of most xero- 

 phytes being generally in a state of partial closure, while those of hydrophytes and 

 of plants with motile leaves, as well as the air pores of liverworts, generally are open; 

 even in mesophytes, closure by no means implies hermetic sealing. The guard cells, 



