ABSORPTION AND EXHALATION OF GASES BY LEAVES. 801 
none at all. The experiments of Cloez and Gratiolet lead sub- 
stantially to the same conclusions. That is to say, that the rays 
which photographically are most active are almost or wholly 
inert in the decomposition of carbon dioxide and the elimination 
of oxygen, while the so-called non-actinic rays are the most 
active. Some heat is necessary for this decomposition, and 
within certain limits it is found that a slight increase of heat 
will compensate for a corresponding diminution of active light 
rays. (See also The Effect of the Electric Light on the Growth 
of Plants, &c., page 858.) 
Whilst the absorption of carbon dioxide and evolution of 
oxygen are 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 carbon dioxide 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 air, and be Supplied under such circum- 
stances with carbon dioxide, the quantity of oxygen gas in the 
contained air becomes increased. 
Some authors, such as Burnett, Carpenter, and Garreau, 
maintain that carbon dioxide is given off by the leaves in vary- 
ing quantities, both by day and night ; whilst others again, such 
as Pepys, Cloez, and Gratiolet, deny that leaves, at any time 
when in a healthy state, give off carbon dioxide. 
Those, again, who hold the opinion that leaves when ex- 
posed to solar light give off oxygen, in consequence of the 
absorption and decomposition of carbon dioxide, and that a 
contrary change takes place by night, maintain different views 
upon the nature of these changes. Some of them regard the 
evolution of oxygen by day asa true vegetable respiration, and 
hence 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 carbon di- 
oxide, 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 
consist in the absorption of oxygen and evolution of carbon 
dioxide. The former function they regard as a process of as- 
similation, aud the latter as respiration. Broughton has more 
recently demonstrated a constant evolution of carbon dioxide 
from nearly all parts of growing plants, and considers that 
this gas, though partly due to previous oxidation, is mainly 
separated from the proximate principles during chemical 
changes. 
Those who maintain Burnett’s views regard the constant 
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