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, and along with it
descending to the foot-stalk or petiole. On slitting one of these
leaves with scissors, and having a magnifying-glass ready, the
milky blood was seen oozing out of the returning veins on each
side of the red artery in the middle rib, but none of the red