16 SCIENCE PROGRESS. 



they act as valves, allowing gases to pass out when the 

 internal pressure is high. 



In 1S77 and 1878, Merget contributed three short notes 

 to the French Academy opposing Barthe.lemy, and experi- 

 mentally supporting the stomatic role by ingenious though 

 rather inconclusive diffusion experiments. 



The first (7) attempts to follow the path of diffusion into 

 leaves of those vapours whose passage can be traced by 

 their discoloration effects. Of these, mercury vapour is 

 considered to yield the most decisive results, as it is 

 said that it cannot be carried in a water current nor 

 osmose through a colloid membrane. Boussingault (8) 

 and others have shown the strong toxic action of this 

 vapour, and that it particularly attacks protoplasm, pro- 

 ducing dark discoloration and death. If a leaf be exposed 

 to mercury vapour, even the deeply situated cells turn 

 dark in a short time. From this ready penetration by 

 a substance that cannot osmose, Merget argues that the 

 vapour must pass through the stomata and penetrate by 

 the intercellular spaces. Also, if a leaf be injected with 

 mercury and pressed between paper sensitised with AgN0 3 , 

 the blackening of the paper by the vapour only occurs in 

 those parts which bear stomata. The interpretation given 

 is no doubt correct, but it is not logically conclusive, for the 

 very fact that mercury attacks protoplasm shows that it must 

 osmose through a colloid membrane — the cell w r all — and so 

 might enter the leaf partly through the cuticle. One fact 

 the author does establish, namely, that diffusion takes place 

 through stomata with equal facility in either direction. 

 Several short controversial papers by Barthelemy and 

 Merget then appeared without much changing their ori- 

 ginal positions. 



No further papers bearing directly on this subject 

 appeared till 1887, when Mangin published two summary 

 notes (9), (10), and then followed them up in 1888 by a 

 detailed paper (11) of careful experiments on diffusion 

 through cuticle. 



The fundamental object of this paper is the determina- 

 tion, firstly, of the quantity of gas that might diffuse under 



