Chap, ii Photosynthesis 285 



in the air is too small to produce much osmosis through 

 the cuticle. 



After 1887 opinion set in steadily in favour of the stomatal 

 theory. Wiesner and Molisch proved in 1889 that an 

 epidermis with no stomata does not allow of an appreciable 

 mechanical passage of gas under pressure ; Boehm, in his 

 researches on the formation of starch from various sub- 

 stances supplied to the plant, found diffusion possible, but 

 extremely difficult and slow, through the epidermal cells. 



Researches carried out by Stahl in 1894 were based upon 

 the discovery made by Sachs much earlier, that the appear- 

 ance of starch in the chloroplastids immediately follows 

 the appropriation of carbon dioxide, a point which will be 

 discussed a little later. Stahl used the appearance of starch 

 as affording, therefore, qualitative evidence of the absorp- 

 tion of the gas. He examined the behaviour of leaves of 

 numerous plants when certain limited areas of each were 

 coated with wax in such a way as to occlude the stomata. 

 He found a uniform result ; abundant starch formation 

 could be observed in all the normal parts of the leaf, but 

 none was formed in the coated areas, even though they 

 were given a prolonged exposure to light. 



Experiments carried out in the same year by Meissner 

 on somewhat similar lines gave the same results. 



Some researches of Frost Blackman's, carried out at 

 Cambridge, overlapped those of Stahl and Meissner, but 

 were not published till 1895. While dealing with the 

 general question they supplemented some observations made 

 by Godlewski in 1873, and, together with these, afforded an 

 explanation of the results of Boussingault, on which the 

 cuticular hypothesis was founded. 



Blackman's method consisted of enclosing leaves in two 

 specially prepared glass chambers, so arranged that each 

 face communicated with a single chamber and the two 

 chambers were perfectly separated from each other, the 



