ATMOSPHERIC AIR AS THE FvOD OF PLANTS. 25 
have no power to increase in mass at the expense of the 
atmosphere and soil; they have no provision for the ab- 
sorption of the nutritive elements that surround them ex- 
ternally, but grow at the expense of other parts of the plant 
(or seed) to which they belong. The function of free 
gaseous oxygen in vegetable nutrition, so far as can be 
judged from our existing knowledge, consists in effecting 
or aiding to effect. the conversion of the materials which 
the leaves organize or which the roots absorb, into the 
proper tissues of the growing parts. Free oxygen is thus 
probably an agent of assimilation. Certain it is that the 
free oxygen which is absorbed by the plant, or, at least, a 
corresponding quantity, is evolved again, either in the un- 
combined. state or in union with carbon as carbonic acid. 
Exhalation of Oxygen from Foliage.—The relation of 
the leaves and green parts of plants to oxygen gas has 
thus far been purposely left unnoticed. These organs like- 
-wise absorb oxygen, and require its presence in the atmos- 
phere, or, if aquatic, in the water which surrounds them; 
but they also, during their exposure to light, exhale oxygen. 
This interesting fact is illustrated 
by a simple experiment. Fill a 
glass funnel with any kind of fresh 
leaves, and place it, inverted, in a 
wide glass containing water, fig. 
8, so that it shall be completely 
immersed, and displace all air from 
its interior by agitation. Close the 
neck of the funnel air-tight by 
a cork, and pour off a portion 
of the water from the outer vessel. Expose now the 
leaves to strong sunlight. Observe that very soon minute 
bubbles of air will gather on the leaves. These will 
gradually increase in size and detach themselves, and 
after an hour or two, enough gas will accumulate in 
the neck of the funnel to enable the experimenter to 
2 
Fig. 3. 
