CHAP, ii Photosynthesis 287 



decomposes less. The great amount of carbon dioxide 

 absorbed by the stomata under a high pressure of the gas 

 acts as a poison to the chlorenchymatous cells. 



Earlier researches of Godlewski's, made in 1873, show 

 that large quantities or high pressures of carbon dioxide 

 in the air prevent the formation of starch, which, as we 

 shall see later, is a sign of the absorption of the gas. 



In the light of these researches it is evident that Boussin- 

 gault's conclusions cannot be accepted as indicating what 

 takes place in a normal leaf. When carbon dioxide is 

 supplied in too concentrated a mixture, and consequently 

 at an altogether abnormal pressure, sufficient can diffuse 

 through the cuticle to give apparently normal appropria- 

 tion. With approximately normal pressure, no appreciable 

 diffusion takes place. 



It remained for Brown and Escombe to show, in 1900, 

 that it is possible for carbon dioxide, present in as low a pro- 

 portion as -026 per cent, in the air, to enter a leaf through 

 such minute apertures as stomata in quantities so great 

 that a sunflower is able to manufacture 1-8 gram of carbo- 

 hydrate per square metre of surface per hour. In a series 

 of experiments on the passage of carbon dioxide through 

 diaphragms pierced by minute apertures they found, if the 

 latter are sufficiently small, diffusion takes place as rapidly 

 as if there were no separating partition at all. This result 

 was reached when the individual openings were sufficiently 

 far apart to be unable to influence each other's activity, 

 which was the case when the distance between them was 

 at least ten times the diameter of the aperture. Under 

 favourable conditions for subsequent photosynthetic 

 activity, and within certain limits of concentration of 

 the carbon dioxide, the absorption by a leaf proved to 

 be proportional to the partial pressure of the gas. In 

 other words, there is an approximate proportionality 

 between the photosynthetic work a leaf can accomplish 



