i 4 SCIENCE PROGRESS. 



Among other results he arrived at the well-known one 

 that transpiration of water-vapour takes place to a much 

 greater degree through areas provided with stomata than 

 through uninterrupted cuticle, so that more water is given 

 off from the lower than from the upper surface in the great 

 majority of leaves. 



In the same paper in which these often-quoted experi- 

 ments are detailed, there is a usually neglected section 

 which bears immediately on the subject now under con- 

 sideration. In this Garreau gives the results of the 

 extension of his method to the comparison of the amounts 

 of carbon dioxide given off by the two surfaces of leaves 

 respectively. 



He attached a glass chamber containing a small shallow 

 capsule of lime water to corresponding areas on each surface 

 of a leaf. The carbon dioxide liberated by the leaf produced 

 a film of calcium carbonate, of varying thickness, on the 

 surface of the lime water ; this was too fragile for direct 

 weighing, so he collected it on pieces of tared filter-paper, 

 and after drying determined the gain in weight. His 

 estimations are all too high, probably increased by C0 2 

 from the air, and the method is of course decidedly in- 

 adequate ; but the results tended strongly to show some 

 relation between the distribution of stomata and the evolu- 

 tion of carbon dioxide, and, among the conclusions to the 

 paper, we find it stated that the expired quantities of carbon 

 dioxide appear to be in a much closer relation to the dis- 

 tribution of stomata than do those of exhaled water. No 

 attempt to obtain similar numbers has been published during 

 the time that has elapsed between the paper under con- 

 sideration and the present author's work. 



In 1867, Boussingault contributed a long experimental 

 paper (3). He directed his attention entirely to the gaseous 

 exchange taking place during active assimilation. As this 

 exchange far exceeds in intensity that of respiration, the 

 former subject appears much the more likely to give a 

 decisive experimental answer to the problem of the nature 

 of the path of exchange. 



The results, however, led him to take up a view exactly 



