738 PHYSIOLOGY. 



therefore, in pure distilled water, or in that which has been 

 recently boiled. It has been found also, that there is a constant 

 relation between the amount of carbonic acid gas absorbed 

 and the oxygen exhaled. These experiments prove therefore, 

 not only the exhalation of oxygen by the leaves, but also that it 

 must be derived by the decomposition of the absorbed carbonic 

 acid. These changes do not take place in the deep-seated 

 tissues of the plant, nor in the epidermal cells, but in those 

 immediately beneath them. 



This absorption of carbonic acid with fixation of carbon 

 and evolution of oxygen, is in direct proportion to the inten- 

 sity of the light to Avhich the plants are exposed ; but the expe- 

 riments of Draper, Hunt, and others, show that the different 

 rays of the spectrum have a varying influence in promoting 

 such a decomposition. The results obtained by Draper by 

 exposing the green parts of plants to the different rays of the 

 spectrum were, that no oxygen was set free by them when they 

 were in the violet and indigo rays; '00 to "SS only when in the 

 extreme red ray; 1 in the blue; 4-10 in the green and blue; 

 43-75 in the yellow and green; and 24-75 in the red and orange. 

 Hence he concluded, that the illuminating rays have the 

 greatest effect in promoting decomposition of carbonic acid, 

 those nearest them much less so, and the heating and chemical 

 rays none at all. The experiments of Cloez and Gratiolet lead 

 substantially to the same conclusions. 



Whilst the absorption of carbonic acid and evolution of 

 oxygen is thus taking place by day, it is supposed by most 

 observers, that in the absence of light, a contrary change takes 

 place, — oxygen being then absorbed, and carbonic acid exhaled. 

 At the same time, all who hold this opinion admit, that the 

 amount of oxygen gas thus absorbed by night, is very much less 

 than that given off by day. Thus, the experiments of Saussure 

 and Daubeny prove, that if plants be enclosed in jars containing 

 ordinary atmospheric acid, and be supplied under such circum- 

 stances with carbonic acid, that the quantity of oxygen gas in 

 the contained air becomes increased. 



Some authors, such as Burnett, Carpenter, and Garrcau, 

 maintain that carbonic acid is given off by the leaves in varying 

 quantities, both by day and night; whilst others again, such as 

 Pepys, and Cloez and Gratiolet, deny that leaves, at any time 

 when in a healthy state, give off carl)onic acid. 



Those who liold the more generally received opinion — that 

 leaves when exposed to solar light give off oxygen gas, in con- 

 seciucncc of tlic absorption and decomi)osition of carbonic acid, 

 and tliat a contrary ciiangc takes i)lace by night ; take different 

 views upon the luiture of these changes. Most of them regard 

 the cvohition of oxy<:cn by day as a true vegetable resi)iration, 

 and hence they look upon vegetable respiration as affording 



