Chap, ii Photosynthesis 283 



posed 17-5 c.c. of carbon dioxide, while the leaf with 

 them open only decomposed 10 c.c. The amount of 

 carbon dioxide in the air in the glass tubes was about 

 30 per cent. 



The inference that Boussingault drew was that the mode 

 of entry of the carbon dioxide into the leaf is a process 

 of diffusion through the epidermis into the interior ; the 

 upper surface being more actively concerned than the 

 lower. 



The more definite suggestion that the cuticle is the part 

 of the epidermis which is the important factor was due 

 to Barthelemy, who brought forward the results of his 

 experiments in support of Boussingault in 1868. He held 

 that the constitution of the cuticular layers of the epidermal 

 cell walls shows a special fitness for transmitting carbon 

 dioxide into the leaf, approximating as it does, according to 

 Fremy, to the constitution of caoutchouc. 



Barthelemy thought that the function of the stomata 

 is valvular, and that they regulate the exit of gases 

 from the plant, but do not allow them to enter. Like 

 Boussingault, therefore, he denied to them any share in 

 the process of absorption of carbon dioxide. 



Barthelemy 's views were opposed by N. J. C. Miiller in 

 1870 and 1873 ; the experiments conducted by the latter 

 being upon the same lines but carried out with greater 

 exactness. Another paper by the former author appeared 

 in 1874, in which he brought forward further evidence in 

 support of his original position, and affirmed it with 

 greater emphasis than before. 



He was replied to in 1877 and 1878 by Merget on the 

 basis of experiments made by exposing leaves to the vapour 

 of mercury, which, being incapable of diffusion, can only 

 enter through the stomatal apertures. Merget found the 

 effects of the vapour very speedily noticeable in the interior 

 of the leaf. 



