ABSORPTION OF GASES. 79 



With regard to the absorption of oxygen by plants and 

 parts of plants which contain chlorophyll abundantly, Ingen- 

 housz pointed out that they convert a considerable portion 

 of the air surrounding them into carbon dioxide when they 

 are not exposed to light. De Saussure found that leaves of 

 the Oak, the Acacia, the Spanish Chestnut, and other plants, 

 if left during the night in a receiver containing air over 

 mercury, absorbed oxygen. Boussingault has also found 

 that leaves absorb oxygen in darkness. 



Subjoined are some of the quantitative results which de Saussure 

 obtained : in each case the experiment lasted 24 hours, and took place in 

 the dark : the quantities of oxygen absorbed are expressed in terms of the 

 volume of the leaf taken as unity. 



Holly (September) o'S6 vol. O. 



Box 1-46 



Cherry Laurel (May young leaves) 3*20 



,, (September) 1-36 



Beech (August) 8'oo 



Oak (May) 5-50 



Poplar (May) 6*20 



(September) 4-36 



From these figures it appears further that young leaves absorb more 

 actively than older ones. 



Since it was known that, as we shall see hereafter, leaves 

 and other parts of plants which contain chlorophyll absorb 

 carbon dioxide and give off oxygen when they are exposed 

 to sunlight, it was concluded that an absorption of oxygen 

 takes place only in darkness. It was thought that during the 

 day green plants absorbed carbon dioxide and gave off 

 oxygen, whereas at night they absorbed oxygen and gave off 

 carbon dioxide. The researches of Garreau shewed, how- 

 ever, that this view was not correct. He came to the conclu- 

 sion that, in leaves exposed to the light, two distinct processes 

 are in operation, in the one oxygen is absorbed and carbon 

 dioxide is exhaled, in the other carbon dioxide is absorbed 

 and oxygen is exhaled. When the light is intense, when the 

 leaves are exposed to bright sunlight for instance, the relative 

 activity of the latter of these processes is so much greater than 



