344 



FACTORS AFFECTING PHOTOSYNTHESIS 



the epidermis. Critical experiments have shown, however, that the propor- 

 tion of this gas entering leaves by this route is relatively small, and that 

 practically all of the carbon dioxide entering leaves diffuses in through the 

 stomates. 



The rate of entrance of carbon dioxide is very considerable in propor- 

 tion to the aggregate area of the stomatal pores. Under conditions favorable 

 for photosynthesis Brown and Escombe (1900) found that carbon dioxide 

 would diffuse into a catalpa leaf from the atmosphere at the rate of 0.07 cc. 

 per square centimeter of leaf surface per hour. Since the stomates in this leaf 

 occupy only 0.9 per cent of surface, diffusion of carbon dioxide gas through 

 them took place at a rate of 7.77 cc. per square centimeter of stomatal 

 aperture per hour. Under the same conditions a normal solution of sodium 

 hydroxide absorbs from the atmosphere, even in rapidly moving air, only 

 0.177 cc. of carbon dioxide per square centimeter per hour. In other words 

 carbon dioxide gas diffuses through the stomates at a rate approximately fifty 

 times as fast as it diffuses into an eflRcient absorbing surface. 



The extremely rapid rate of diffusion of carbon dioxide gas through the 

 stomates can be interpreted in the light of the principles of diffusion of gases 

 through small openings. Table 33, condensed from the work of Brown 

 and Escombe, indicates that these same principles hold, in general, for carbon 

 dioxide as well as water-vapor (Chap. XIII.) 



TABLE 2>3 DIFFUSION OF CARBON DIOXIDE THROUGH MULTIPERFORATE SEPTA. PORES 



0.380 MM. IN DIAMETER; SEPTA I CM. FROM SODIUM HYDROXIDE SOLUTION (dATA OF 

 BROWN AND ESCOMBE, I9O0). 



The most important principle illustrated, that the rate of diffusion does 

 not decrease proportionately with reduction in the aggregate area of the pores, 

 is shown in the last two columns. For septum 4 (pores 10.52 diameters 



