NUTRITION 49 



Cell-walls which are gelatinized or cutinized and infiltrated 

 with waxy matters, as are those of the epidermal cells of 

 Fucus, are less permeable to gases than are cellulose walls. 

 Through the cryptostomata of Fucus, therefore, as well as 

 through the stomata of higher land plants, the inward 

 diffusion of carbon-dioxide is more rapid than through the 

 walls of the epidermal cells. 



For land-plants the value of the stomata as furnishing the 

 gates through which the exchange of gases takes place, has 

 been strikingly demonstrated by Stahl. * He shows that by 

 smearing the stomata-bearing surface of leaves having 

 stomata on one side only (usually the under side) with 

 cocoa-butter or vaseline (neither of which is irritating), the 

 absorption of carbon-dioxide, as indicated by the amount 

 of starch formed, will be much less than when the gas can 

 diffuse through both the open stomata and the more perme- 

 able walls of the epidermal cells of the stomata-bearing sur- 

 face. If the epidermal cells of the upper surface of a leaf 

 which has stomata only on the under and smeared side are 

 injured by a cut or scratch just deep enough to penetrate 

 the wall, the much more rapid diffusion of carbon-dioxide 

 through the cut into the leaf than through the walls 

 of uninjured epidermal cells will be shown by the greater 

 quantity of starch formed in the chlorophyll-containing 

 cells adjacent to the cut. In green land-plants the ab- 

 sorption of carbon-dioxide into the intercellular spaces is 

 accomplished by the free inward diffusion of the gas, a 

 diffusion which is controlled by the stomata, which can 

 open or close as occasion demands. From these intercell- 

 ular spaces the green cells absorb what carbon-dioxide 

 they need through their permeable cellulose walls, just as 

 the luwer algae absorb all their food-materials through 

 their permeable cell-walls. 



As we have already seen (p. 45), the volume of carbon- 

 dioxide absorbed implies the very considerable absorbent 

 power of the leaf, but when we recall how small a propor- 

 tion of the leaf surface (e. g. \% in Catalpa bignonioides) 



* Stahl, E. Einige Versuche iiber Transpiration und Assimilation. 

 Botanische Zeitung, Bd. 52, 1894. 

 4. 



