BRIGGSI THE LIVING PLANT AS A PHYSICAL SYSTEM 101 



leaf are extremely minute and their total area is only about 1 

 per cent of the leaf area. How does the carbon-dioxide enter 

 the leaf? Does the interchange of gas and water vapor between 

 the leaves and the air take place by means of diffusion through 

 the cuticle of the leaf? Or do the minute stomatal openings 

 provide the means of entrance and exit? 



The early experiments of Boussingault, 5 in which the rate of 

 assimilation of carbon-dioxide by leaves having their stomata 

 blocked with lard was compared with that of normal leaves, led 

 to the conclusion that the absorption of carbon-dioxide by the 

 leaf takes place by means of cuticular diffusion, and that the 

 stomata play little or no part in the process. The cuticular 

 diffusion hypothesis found favor also on account of the relatively 

 large surface available for diffusion compared with the area of 

 the stomatal openings. 



Boussingault's experiments, however, were carried out in an 

 atmosphere abnormally high in carbon dioxide, and in 1895, 

 Blackman 6 showed that the relative amount of carbon dioxide 

 assimilated by leaves with stomata blocked and free was depen- 

 dent upon the carbon dioxide content of the air. When the 

 carbon dioxide content was reduced to partial pressures corre- 

 sponding more nearly with atmospheric conditions, an Oleander 

 leaf with its stomata blocked showed a much lower assimilation 

 of carbon dioxide than a similar leaf with open stomata. In 

 other words, the high partial pressure of carbon dioxide in Bous- 

 singault's experiments resulted in a carbon dioxide poisoning 

 of the leaf having open stomata, and a corresponding reduction 

 in assimilation; whereas, in the case of the leaf with its stomata 

 blocked, sufficient protection was afforded to permit assimilation 

 to proceed normally. Blackman concluded that for partial 

 pressures of carbon dioxide approaching that in the atmosphere, 

 no appreciable diffusion through the cuticle of the leaf takes 

 place. 



6 Boussingault, M. Etude sur les fonctions des feuilles. Agronomie, Chemie 

 Agricole et Physiologie, 4: 267-401. 1868. 



6 Blackman, F. F. On the paths of gaseous exchange between aerial leaves and 

 the atmosphere. Phil. Trans., 186b: 508-562. 1895. 



