136 THE LIFE OF THE PLANT 



in extracting from the atmosphere the meagre propor- 

 tion of carbonic acid contained in it, until it was dis- 

 covered that, owing to peculiarities in the diffusion 

 of gases, demonstrated for the first time in Brown's 

 experiments, carbonic acid enters the leaf through 

 these stomatal apertures almost in the same quantity as 

 it would diffuse had the whole surface of the leaf taken 

 part in the process of absorption. Horace Brown wittily 

 remarked, in commenting upon this remarkable dis- 

 covery, that a plant evidently knows more about physics 

 than we are inclined to admit ! His experiments also 

 showed that if the percentage of carbonic acid in the air 

 were increased five times, from the usual proportion of 

 I0 ooo to TuV<7, the quantity decomposed by the plant 

 also increased to about five times as much ; a result 

 which justifies us in concluding with greater certainty 

 than before, how successfully the plant can use up 

 the atmospheric carbonic acid, seemingly available 

 in such minute quantities. 



On coming into contact with a green plant in sun- 

 light carbonic acid is decomposed : its oxygen is set free, 

 while its carbon is deposited in the plant. Let us try 

 to trace the further fate of this carbon inside the plant. 

 Let us again turn to the microscope. Almost without 

 exception every observation of this phenomenon, and 

 every experiment, point to the conclusion that this 

 process takes place only in the green parts of the plant. 

 We can really say with certainty that if an organ is 

 not green, it does not decompose carbonic acid ; while 

 if it does decompose it, the green colouring matter when 

 not directly visible is hidden by other colouring sub- 

 stances. These green granules, which contain this green 

 colouring matter, called chlorophyll, serve as organs in 

 which the decomposition of carbonic acid takes place. 



