INFUSION OF CARBON DIOXIDE 21 



over the surface, though unimportant for a single stoma, 

 may be important when there are thousands spread over a 

 considerable area." If there is no interference between the 

 density shells of stomata ten diameters or more apart, so that 

 each stomate acts independently according to the law of 

 diameters, an impossible situation results. Thus if there are 

 more than 600 stomata per sq. cm. of leaf surface, the rate 

 of evaporation from them, which is the same thing as carbon 

 dioxide diffusion, would be greater than that from the whole 

 leaf surface, wherefore it must be concluded that gaseous 

 diffusion must be greatly impeded by the presence of other 

 stomata. It is not until stomata contract to one-fiftieth of 

 their original diameters, that the rate of evaporation, or gaseous 

 diffusion, will be practically independent of the diameter. 

 When the air is in motion and the stomata are fully open, 

 their total evaporation, or absorption of carbon dioxide, does 

 not differ much from the evaporation, or absorption, from an 

 area equal to the leaf surface ; * it is only when the number of 

 stomates per unit area is small, that each may act indepen- 

 dently, and the law of diameters obtain. 



The conditions of humidity of atmosphere, temperature 

 and illumination, which affect the size of the stomates, ob- 

 viously will affect the infusion rate of carbon dioxide. f In 

 addition to these movements, traceable to definite external 

 causes, there is sometimes — the leaf of the cherry laurel, 

 for instance — a diurnal rhythm of stomatal opening even when 

 the leaf is grown in controlled conditions and constantly 

 illuminated. The result of this is that in a normal atmos- 

 phere, when the leaf is in constant light, there is a diurnal 

 rhythm in the carbon assimilation, the rate of which falls to 

 a very low value during the night and rises again next morning. 

 This diurnal, and also seasonal, variation in the assimilation 

 rate runs parallel with the diurnal and seasonal rhythms of 

 stomatal openings. It is only when the external atmosphere 



* Cf. Renner : " Flora," 1910, 100, 451. 



f See Darwin : " Phil. Trans. Roy. Soc," B, 1898, 190, 531 ; 1916, 

 207, 413. Lloyd : " Carnegie Inst. Washington," No. 82, 1908. Knight : 

 " Ann. Bot.," 1916, 30, 57. 



