286 The Physiology of Plants BOOK in 



leaf forming a diaphragm between them. A tube led to 

 each chamber from a reservoir containing air supplied with 

 a known percentage of carbon dioxide, usually about I per 

 cent., and another led from the chamber to a titration 

 apparatus specially devised for the measurement of very 

 small quantities of the gas. Each leaf was carefully 

 examined with a lens to ensure that no perforations in its 

 surface existed. A stream of air was drawn through the 

 apparatus for a definite time by an aspirator under proper 

 precautions to ensure a regular and measurable flow. The 

 light was admitted to the leaves by exposing the apparatus 

 either to direct sunlight or to the reflected light from 

 a heliostat, and was made to fall upon the leaves at right 

 angles to their surface. Measurements were usually taken 

 of the quantities of gas passed over the surfaces of the 

 leaves during fifteen minutes. The leaves used were those 

 of Ampelopsis hederacea, Platanus occidentalis, and Poly- 

 gonum sachalinense, with stomata on the lower surface 

 only; and Alisma Plantago and Tropaeolum majus, whose 

 surfaces are both stomatiferous. The results showed that 

 the absorption of carbon dioxide follows extremely closely 

 the distribution of the stomata ; in fact, in the cases of 

 the leaves of the first three plants mentioned, there is no 

 intake of the gas by the upper surfaces. 



Blackman subsequently repeated Boussingault's ex- 

 periments, using, as he did, the leaves of the Oleander 

 and passing over them a mixture of air and carbon 

 dioxide of the same composition, viz. one containing 30 per 

 cent, of the latter. He found that he confirmed Boussin- 

 gault's results in all essential particulars. He then used 

 mixtures of the two in different proportions and obtained, 

 with low percentages of carbon dioxide, exactly opposite 

 results. With a high percentage of the gas, then, he deter- 

 mined that a leaf with occluded stomata always decomposes 

 more than a normal leaf, while with low percentages it 



