PHOTOSYNTHESIS 211 



that the error into which Boussingault had fallen was due 

 to his having used far too high percentages of carbon 

 dioxide. Blackman's results thus confirmed and greatly 

 extended the conclusions of Mangin, announced some 

 ten years previously. Blackman also showed that the 

 distribution of stomata on leaves of different plants 

 corresponded practically exactly with the volumes of 

 gas passing into and out of the leaf, and thus completely 

 set at rest the controversy that had been in progress 

 since Boussingault's day. 



Another very important piece of work was carried 

 out in 1900 on the same subject by Brown and Escombe, 

 in which the authors showed that stomata, as a general 

 rule, were placed at intervals apart which were about 

 eight to ten times the average diameter of the stomatal 

 opening, and that this distribution coincided with the 

 conditions which must be fulfilled under the law of 

 diffusion of gases through perforated membranes. Wiesner, 

 in 1879, ^^^ rnade some observations of the same nature 

 but without the finished detail that is so marked a feature 

 of Brown and Escombe's work. I must leave 3^ou to 

 read for yourselves this highly important and convincing 

 research ; to attempt to condense it would destroy the 

 beautifully logical mode of presentation in which the 

 authors' conclusions are worked out. 



Those of you who are chemists, and are familiar with 

 Roscoe and Schorlemmer's great Treatise on Chemistry, 

 will notice that I have not referred to the many papers 

 that appeared during these forty years on the chemistry 

 of chlorophyll, and that I have omitted all mention of 

 such well-known names as those of Marchlewski, Schunck, 

 Tswett, Tschirch, and others who investigated the 

 chemical composition of the green pigment by all con- 

 ceivable methods, and with results of the most varied 

 and often contradictory nature. This omission is inten- 

 tional, for if I attempted to give you some idea of the 

 labours of one investigator I should have to deal with 



