74 THE CHEMICAL ACTION OF LIGHT. 
is absorbed by the green parts from the air, and is decomposed 
by the chlorophyll under the influence of light, setting free 
oxygen. The chlorophyll is almost universally associated with 
starch-granules, and it has been found that the presence of the 
latter is dependent upon exposure to light. The formation of 
starch, in fact, depends upon the same conditions as does the 
decomposition of carbonic acid, and is intimately connected with 
it, as is shown by the fact that if leaves exposed to light are not 
supplied with carbonic acid no starch-granules are formed in the 
chlorophyll corpuscles. We see, therefore, that the formation of 
starch-granules is the visible product of the absorption of carbonic 
acid by plants. 
But if green leaves under the influence of light absorb carbonic 
acid and give off oxygen, in darkness they do the very reverse— 
oxygen being absorbed and carbonic acid given off. Owing to 
this well-marked difference, it was formerly thought that during 
the day green plants absorbed carbonic acid and gave off oxygen, 
whilst at night the contrary was the case. We now know, how- 
ever, that this view was not correct. In plants exposed to the 
light the two processes are always going on side by side. On the 
one hand, oxygen is absorbed and carbonic acid exhaled ; on the 
other hand, carbonic acid is inhaled and oxygen is given off. 
When the light is intense, as for instance when leaves are exposed 
to bright sunlight, the relative activity of the latter of these 
processes is so much greater than that of the former that it alone 
appears to be in operation. 
I have stated that a green plant is incapable of constructing 
organic substance from the materials of its food unless it is 
exposed to light, and that under these conditions it increases in 
weight; but when in darkness, the plant not only does not 
increase in weight, but it even loses weight in consequence of 
the exhalation of carbonic acid and the loss of aqueous vapour 
in respiration. Prolonged exposure to darkness must therefore 
eventually prove fatal to the plant, the length of time required 
being determined by the amount of reserve material which the 
plant possesses. On the other hand, adequate exposure to light 
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