the lungs or air-vessels in men is said to
be equal to the external surface of the whole body, or almost fifteen
square feet; on this surface the blood is exposed to the influence of
the respired air through the medium, however, of a thin pellicle; by
this exposure to the air it has its color changed from deep red to
bright scarlet, and acquires something so necessary to the existence of
life that we can live scarcely a minute without this wonderful process.

"The analogy between the leaves of plants and the lungs or gills of
animals seems to embrace so many circumstances that we can scarcely
withhold our consent to their performing similar offices.

"1. The great surface of leaves compared to that of the trunk and
branches of trees is such that it would seem to be an organ well adapted
for the purpose of exposing the vegetable juices to the influence of the
air; this, however, we shall see afterwards is probably performed only
by their upper surfaces, yet even in this case the surface of the leaves
in general bear a greater proportion to the surface of the tree than the
lungs of animals to their external surfaces.

"2. In the lung of animals the blood, after having been exposed to the
air in the extremities of the pulmonary artery, is changed in color
from deep red to bright scarlet, and certainly in some of its essential
properties it is then collected by the pulmonary vein and returned
to the heart. To show a similarity of circumstances in the leaves of
plants, the following experiment was made, June 24, 1781. A stalk with
leaves and seed-vessels of large spurge (Euphorbia helioscopia) had been
several days placed in a decoction of madder (Rubia tinctorum) so that
the lower part of the stem and two of the undermost leaves were immersed
in it. After having washed the immersed leaves in clear water I could
readily discover the color of the madder passing along the middle rib
of each leaf. The red artery was beautifully visible on the under and on
the upper surface of the leaf; but on the upper side many red branches
were seen going from it to the extremities of the leaf, which on the
other side were not visible except by looking through it against the
light. On this under side a system of branching vessels carrying a
pale milky fluid were seen coming from the extremities of the leaf, and
covering the whole under side of it, and joining two large veins, one
on each side of the red artery in the middle rib of the leaf,