ADJUSTMENT TO WATER 61 



two pairs of valves, though these are often found in other stomata 

 also. As already indicated, stomata are ordinarily more abundant 

 upon the lower epidermis of horizontal leaves, and in some species 

 are restricted to this surface. In many aquatic and rosette plants, 

 the stomata are more abundant upon the upper surface, and in 

 floating plants they occur on this surface alone. The leaves of sub- 

 merged plants normally lack stomata. In forms more recently 

 submerged, the latter sometimes persist, but are functionless. 



78. The functions of stomata. In their simplest form, stomata 

 are for the purpose of permitting the ingress and egress of carbon 

 dioxide and oxygen, though moisture must also pass out through 

 them. In the thalloid liverworts, growing closely pressed upon 

 moist earth, the danger of drying out through the openings alone 

 would seem small. In leafy stemmed plants, this danger is greatly 

 increased, and has necessitated the development of guard-cells. 

 The latter consequently have charge of the secondary function of 

 stomata, which is to regulate the amount of transpiration. The 

 movements of the guard-cells are regulated by light and by the 

 interaction of humidity and water content. Stomata open in 

 strong light and close in weak light; consequently they show a 

 periodic movement, opening in the morning and closing at night. 

 When transpiration tends to exceed absorption, as in the case of 

 a great decrease in humidity, the guard-cells close. This checks 

 transpiration, and usually enables the roots to meet the deficiency. 

 When this occurs, or when the water supply is renewed, as by a 

 rain, the guard-cells open. 



79. Movements of guard-cells. The movements of the guard- 

 cells are brought about by changes in their turgidity. Stomata 

 close when the plant becomes flaccid, i.e., when the plant is losing 

 more water than it absorbs. They open again when the plant 

 becomes turgid in response to increased absorption or decreased 

 transpiration. Generally speaking, stomata are open for the 

 exchange of gases, when the danger from excessive water loss is 

 slight, and they remain closed when the danger is great. As a 

 matter of fact, the closure is rarely quite perfect, so that some 

 moisture escapes even when the stoma is closed. 



The mechanism by which the stomata open and close is a simple 

 one. It is most readily understood by comparing the top of a 

 stoma with a cross-section of one. The thinnest wall of each 

 guard-cell is the one next the epidermal cell, the others, particularly 



