1894.] on Vegetable Assimilation and Respiration. 165 



II. " Experimental Researches on Vegetable Assimilation and 

 Respiration. No. II. On the Paths of Gaseous Exchange 

 between Aerial Leaves and the Atmosphere." By F. F. 

 BLACKMAN, B.Sc., B.A., St. John's College, Demonstrator 

 of Botany in the University of Cambridge. Communicated 

 by FRANCIS DARWIN, F.R.S. Received November 15, 1894. 



(Abstract.) 



On the question of the path by which carbonic acid passes out of 

 the leaf in respiration and into it in assimilation, whether this takes 

 place by the stomatal openings or through the continuous surface of 

 the cuticle, all possible extreme and intermediate views have been 

 expressed in recent text-books of botany. On account of the small- 

 ness of the quantities of gas involved, practically no attempt has 

 hitherto been made to determine this question by direct estimation. 

 The existing experimental evidence is all of an indirect nature, and 

 tends rather to support the view that the exchange is a cuticulur 

 phenomenon. 



An ingenious synthesis of Graham's observations on the compara- 

 tive readiness with which C0 2 osmoses through thin films of 

 caoutchouc, with observations by Fremy and others on the similarity 

 between cuticle and caoutchouc in chemical composition, first led 

 Barthelemy (1868) to put forward the view that the cuticle was 

 specially adapted for transmitting C0 2 from the external air to the 

 assimilating cells beneath it. This view he supported by experi- 

 ments on the artificial osmosis of gases through leaves. About the 

 same time Boussingault performed experiments that seemed to 

 definitely show that in assimilation, the C0 2 taken up by the leaf 

 entered it through the upper surface, devoid of stomata, to which 

 the assimilating cells are adjacent, rather than through the more 

 distant stomatal openings. These experiments have hitherto been 

 generally accepted, but I shall show later that the conclusions drawn 

 from them are entirely fallacious. In support of the view that 

 stomata forni the paths of gaseous exchange, besides scattered induc- 

 tions by various workers, we have the conclusion arrived at in 1888 

 by Mangin, from diffusion experiments on isolated cuticle, that this 

 diffusion is insufficient to account for the whole gaseous exchange of 

 the leaf. 



By the aid of the apparatus described in a previous paper, the 

 author has been able successfully to attack the problem directly by 

 estimating the amounts of CO 2 given out or taken in by the two 

 surfaces of the saiiie leaf, under the same conditions. For this pur- 



