752 PHYSIOLOGY. 



the epidermal cells, but in those only immediately beneath the 

 latter. 



This absorption of carbonic acid with fixation of carbon and 

 evolution of oxygen is in direct proportion to the intensity of 

 the light to which the plants are exposed ; but the experiments 

 of Draper, Hunt, and others, sliuw that the different rays of the 

 spectrum have a varying influence in promoting such a decom- 

 position. 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 ; 

 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 con- 

 cluded, 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 action occurs 

 — 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 proA'e, that if plants be enclosed in jars containing 

 ordinary atmospheric air, and be supplied under such circum- 

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

 contained air becomes increased. 



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

 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 Grratiolet, deny that leaves, at any time 

 when in a healthy state, give off carbonic acid. 



Those who hold the more generally received opinion — that 

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

 sequence of the absorption and decomposition of carbonic acid, 

 and that a contrary change takes place by night — maintain 

 different views upon the nature of these changes. Most of them 

 regard the evolution of oxygen by day as a true vegetable respira- 

 tion, and hence they look upon vegetable respiration as producing 

 results upon the atmosphere we breathe diametrically opposite 

 to those of animal respiration Others, such as Mohl and 

 Henfrey, say that here we have two distinct functions going on, — 

 one, taking place by day, and consisting in the consumption of 

 carbonic acid, with fixation of carbon and evolution of oxygen ; 

 and another, only occurring by night, in the leaves and other 

 green parts, but also by night and day, in those not green, and 

 which consists in the absorption of oxygen and evolution of 



