30 SCIENCE PROGRESS. 



numbers experimentally obtained for this function really 

 express the external optimum of carbon dioxide content — a 

 number very convenient to know, but not of direct physio- 

 logical significance. The internal optimum for the atmo- 

 sphere surrounding the assimilating cells themselves must 

 be a smaller number, which might well be constant for all 

 cases. The external optimal number would then be a 

 combined expression of this constant and a variable, which 

 would depend on the structure of the leaf in question, and 

 would be large for those leaves which, by reason either of 

 the smallest or scarcity of their stomata, the narrowness of 

 their intercellular spaces or the thickness of the leaf, offered 

 considerable resistance to the diffusion of carbon dioxide. 



How variable the size and numbers of stomata may be, 

 the determinations given by Weiss (18) plentifully exhibit, 

 and Unger (19) has shown that the volume of the inter- 

 cellular spaces in leaves may vary from 3 per cent, to 71 

 per cent, of the total volume of the leaf. Whether all this 

 variation really does produce a marked effect on the pene- 

 trability of leaves we cannot yet say. 



As a corollary to the experiments just given, mention 

 must be made of the effect of exposing leaves that have 

 their stomata blocked with vaseline to so small a percentage 

 of carbon dioxide as that occurring in the atmosphere. Here 

 the difference between the vaselined leaf and the normal leaf 

 is very marked, and, according to the usual phraseology, the 

 vaselined leaf does not assimilate at all, that is to say, it 

 forms no starch on exposure to bright light. 



If a leaf with its stomata all on the lower surface be 

 freed from starch by darkness, and then, with half the under 

 surface coated with vaseline, be exposed for a whole day in 

 a good light, no starch will be found in this half, while the 

 other has formed starch freely. Coating the upper surface 

 has no effect on starch formation. I performed such ex- 

 periments in the summer of 1893 and showed them to 

 several botanists. Nearly at the close of my research, in 

 July, 1894, Stahl (13) published similar experiments. He 

 found a mixture of cocoa-butter and bees-wax preferable to 

 vaseline. His observations are much more extended than 

 mine, and he gives some interesting figures of the starch 



