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 fluid from the
artery.

"All these appearances were more easily seen in a leaf of Picris treated
in the same manner; for in this milky plant the stems and middle rib of
the leaves are sometimes naturally colored reddish, and hence the color
of the madder seemed to pass farther into the ramifications of their
leaf-arteries, and was there beautifully visible with the returning
branches of milky veins on each side."

Darwin now goes on to draw an incorrect inference from his observations:

"3. From these experiments," he says, "the upper surface of the leaf
appeared to be the immediate organ of respiration, because the colored
fluid was carried to the extremities of the leaf by vessels most
conspicuous on the upper surface, and there changed into a milky fluid,
which is the blood of the plant, and then returned by concomitant
veins on the under surface, which were seen to ooze when divided with
scissors, and which, in Picris, particularly, render the under surface
of the leaves greatly whiter than the upper one."

But in point of fact, as studies of a later generation were to show, it
is the under surface of the leaf that is most abundantly provided
with stomata, or "breathing-pores." From the stand-point of this later
knowledge, it is of interest to follow our author a little farther,
to illustrate yet more fully the possibility of combining correct
observations with a faulty inference.

"4. As the upper surface of leaves constitutes the organ of respiration,
on which the sap is exposed in the termination of arteries beneath a
thin pellicle to the action of the atmosphere, these surfaces in many
plants strongly repel moisture, as cabbage leaves, whence the particles
of rain lying over their surfaces without touching them, as observed by
Mr. Melville (Essays Literary and Philosophical: Edinburgh), have the
appearance of globules of quicksilver. And hence leaves with the upper
surfaces on water wither as soon as in the dry air, but continue green
for many days if placed with the under surface on water, as appears
in the experiments of Monsieur Bonnet (Usage des Feuilles). Hence some
aquatic plants, as the water-lily (Nymphoea), have the lower s