DIFFUSION OF GASES THROUGH STOMA 99 



then under ordinary conditions these quantities of carbon 

 dioxide can diffuse through the stomata. As a matter of fact, 

 the greatest known rate of absorption of carbon dioxide by 

 the sunflower from ordinary air is that given by Thoday 

 (1910) of 0"i5 c.c. per square centimetre per hour. Browne 

 and Escombe point out that the stomatal system amply pro- 

 vides for a full supply of carbon dioxide, since, in fact, at 

 its highest assimilating power in ordinary air, the sunflower 

 leaf uses only 5 to 6 per cent, of the diffusive capacity of the 

 stomata. The reason they give for the low value found is 

 that diffusion through the cell solutions is very slow, quoting 

 Graham's remark, " liquid diffusion of carbonic acid is a 

 slow process compared with its gaseous diffusion, quite 

 as much as days are to minutes." The reason that more 

 carbon dioxide is not used is, therefore, that it cannot travel 

 more rapidly into the cells. 



Taking the case of transpiration from a sunflower leaf 

 in v/ind, at a temperature of 20° C, and with a fall in vapour 

 pressure from saturation (0*02 atmos.) inside the leaf to one- 

 quarter of that amount in the atmosphere, Browne and 

 Escombe found that diffusion through the stomata could 

 take place at the rate of 0*1730 grm. of water per square 

 centimetre per hour ; the maximum amount obtained 

 experimentally was 0*0275 grm., or one-sixth of the 

 theoretical quantity. Browne and Escombe conclude, 

 therefore, that maximal transpiration can be effected by 

 means of the stomata. 



They do not, in this case, offer an explanation of why the 

 actual amount falls short of the theoretical, and it is difficult 

 to see why it should do so, since at the evaporating surface 

 of the mesophyll cells of a turgid leaf the air must be 

 saturated with water vapour. As we have seen, the transpira- 

 tion from the surface of a sunflower leaf may reach three- 

 fifths of the value of an equal area of free water surface. 

 Renner points out that the value calculated by Browne and 

 Escombe is three times the rate of evaporation from a 

 free water surface of equal area, which is of course impos- 

 sible. Renner concludes that equation (5) requires to be 



