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