ROLE OF THE STOMATA 911 



experiments with paraffined leaves, that gas exchange takes place almost exclusively 

 through the stomata, in the way indicated in figure 27.5. Only under very high pres- 

 sure of carbon dioxide did Blackman observe a slight penetration of the gas through the 

 cuticle. According to St&lfclt (1935), in the free atmosphere, carbon dioxide pene- 

 trates the cuticle at a rate of only between 3 and 6 X 10~^ mole/cm. ^ hr.; the gas flow 

 through the stomata may be as much as one hundred times faster, i. e., of the order of 

 5 X 10"'' mole/cm. 2 hr., despite the fact that their openings occupy only about 0.1% 

 of the total leaf surface. Freeland (1946) found more recently that, in some leaves, the 

 relative rate of passage of carbon dioxide under pressure through the lower and the up- 

 per surface is so low as to suggest predominance of diffusion through the epidermis over 

 passage through the stomata. The thickness of the epidermis may be an important 

 factor in the determination of the relative role of stomata and epidermis as routes for 

 the entry of carbon dioxide into the leaf. 



Ferns and other lower land plants possess no stomata, and therefore must receive 

 all their carbon dioxide supply through the epidermis. Stomata also are absent in 

 aquatic plants and algae, where their main function — regulation of evaporation — is not 

 required. 



Between 10,000 and 30,000 stomata are present on each square centi- 

 meter of the leaf surface, either on both sides or on the lower side only. 

 They are elongated slits, usually from 10 to 15 ix long, flanked by two "guard 

 cells" {cf. figs. 27.5 and 27.6), which are capable of changing shape so as to 

 effect the opening or closing of the slit {cf. fig. 27.7). 



Fig. 27.6. A portion of the lower epidermis of a geranium leaf (after 



Robbins and Rickett). 



This mechanism is brought into operation by shifts in the sugar-starch 

 equilibrium, which increase the turgor when the slits are to be opened, and 

 decrease it when they must be closed. 



The problem of the diffusion resistance of stomata has been considered 

 from two points of view: First, it was asked: Is it possible for a diffusion 

 flow of up to 10-5 mole (0.24 cc.) COa/hr. (c/. chapter 28, Table 28.5) to 

 pass through the stomata on 1 cm.- of the leaf surface, when the total open 



