THE NATURE OF PHOTOSYNTHESIS 77 



cellular spaces, by complete absorption, the amount of gas in cc. absorbed 

 by one sq. cm. of leaf per hour will be represented by: 



kp . A . y . 3600 

 ^ L+x 



k r=: diffusion constant of CO2 = 0.145. 



p ^ density of COo in outer air in atmospheres = 0.0003. 

 A = area of stomata = 10"" X 9.08 cm. 



y = number of stomata per sq. cm. == 33,000. 

 L r^ length of stomatic tube = 0.0014 cm. 



X ^ resistance of the column of diffusive flow = y^Tt X diameter = 



0.00042 cm. 

 Q = 2.578 cc. CO2 per hour per sq. cm. 



If it is supposed that the air over the leaf is perfectly still Brown and 

 Escombe develop the equation so that the denominator becomes L -|- 2x 

 and Q = 2.095. 



A comparison with the actually observed rates of photosynthesis will 

 show that these rates are very much lower than those theoretically 

 possible on the basis of Brown and Escombe's calculations. Prob- 

 ably the chief factor in this discrepancy is the fact that the walls of the 

 intercellular spaces into which the stomata lead are not perfect absorbers 

 of carbon dioxide. In 1850, Graham, ^^ in his Bakerian Lecture, pointed 

 out that the "liquid diffusion of carbonic acid is a slow process compared 

 with its gaseous diffusion, quite as much as days are to minutes." The 

 gradient between the carbon dioxide on the outside and inside of the leaf 

 is therefore much smaller than is assumed in the formula. A considera- 

 tion of the absorptive capacity of the leaf material is taken up in Chapter 5. 



The outstanding contribution of Brown and Escombe's studies is the 

 fact that the structure of a leaf with its minute stomata is admirably 

 adapted to the work it has to perform. The surface of a leaf, with the 

 physical properties of a multi-perforate septum, having only 1 to 3 per 

 cent of open area, still permits free gaseous interchange. The stomata 

 can. in fact, be closed to 5 per cent of their maximum and yet permit 

 sufficient carbon dioxide to pass to account for the maximum photo- 

 synthesis, provided the absorption is perfect. 



This aspect of the photosynthesis problem has received very little 

 attention aside from the very careful studies of Brown and Escombe. 

 It would be highly desirable to have their results verified and extended. 

 Jeffreys ^^ has made a mathematical study of the laws of evaporation 

 of water from circular surfaces and from cylinders. The laws of evapora- 

 tion have a close analogy to those of absorption and from the results 

 of Jeffrey's studies some of Brown and Escombe's conclusions are called 



" Graham, Chemical and Physical Researches, p. 446. 



''Jeffreys, H., Phil. Mag., 35, 270 (1918). Thomas and Ferguson, ibid., 34, 308 

 (1917). 



