absorbing oxygen, or vital air.

"6. The great use of light to vegetation would appear from this
theory to be by disengaging vital air from the water which they
perspire, and thence to facilitate its union with their blood
exposed beneath the thin surface of their leaves; since when pure
air is thus applied it is probable that it can be more readily
absorbed. Hence, in the curious experiments of Dr. Priestley and
Mr. Ingenhouz, some plants purified less air than others--that
is, they perspired less in the sunshine; and Mr. Scheele found
that by putting peas into water which about half covered them
they converted the vital air into fixed air, or carbonic-acid
gas, in the same manner as in animal respiration.

"7. The circulation in the lungs or leaves of plants is very
similar to that of fish. In fish the blood, after having passed
through their gills, does not return to the heart as from the
lungs of air-breathing animals, but the pulmonary vein taking the
structure of an artery after having received the blood from the
gills, which there gains a more florid color, distributes it to
the other parts of their bodies. The same structure occurs in the
livers of fish, whence we see in those animals two circulations
independent of the power of the heart--viz., that beginning at
the termination of the veins of the gills and branching through
the muscles, and that which passes through the liver; both which
are carried on by the action of those respective arteries and
veins."[6]

Darwin is here a trifle fanciful in forcing the analogy between
plants and animals. The circulatory system of plants is really