ides
floating on the water, while the upper surfaces remain dry in the air.

"5. As those insects which have many spiracula, or breathing apertures,
as wasps and flies, are immediately suffocated by pouring oil upon them,
I carefully covered with oil the surfaces of several leaves of phlomis,
of Portugal laurel, and balsams, and though it would not regularly
adhere, I found them all die in a day or two.

"It must be added that many leaves are furnished with muscles about
their foot-stalks, to turn their surfaces to the air or light, as mimosa
or Hedysarum gyrans. From all these analogies I think there can be no
doubt but that leaves of trees are their lungs, giving out a phlogistic
material to the atmosphere, and 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 not quite
so elaborately comparable to that of fishes as he supposed. But the
all-important idea of the uniformit