n's, and it is possible that nyctitropic leaves are
adapted to avoid the indirect as well as the direct results of cooling
by radiation.

In what has been said I have attempted to give an idea of some of the
discoveries brought before the world in the "Power of Movement" (In 1881
Professor Wiesner published his "Das Bewegungsvermogen der Pflanzen",
a book devoted to the criticism of "The Power of Movement in Plants". A
letter to Wiesner, published in "Life and Letters", III. page 336, shows
Darwin's warm appreciation of his critic's work, and of the spirit in
which it is written.) and of the subsequent history of the problems.
We must now pass on to a consideration of the central thesis of the
book,--the relation of circumnutation to the adaptive curvatures of
plants.

Darwin's view is plainly stated on pages 3-4 of the "Power of Movement".
Speaking of circumnutation he says, "In this universally present
movement we have the basis or groundwork for the acquirement, according
to the requirements of the plant, of the most diversified movements."
He then points out that curvatures such as those towards the light or
towards the centre of the earth can be shown to be exaggerations of
circumnutation in the given directions. He finally points out that
the difficulty of conceiving how the capacities of bending in definite
directions were acquired is diminished by his conception. "We know that
there is always movement in progress, and its amplitude, or direction,
or both, have only to be modified for the good of the plant in relation
with internal or external stimuli."

It may at once be allowed that the view here given has not been accepted
by physiologists. The bare fact that circumnutation is a general
property of plants (other than climbing species) is not generally
rejected. But the botanical world is no nearer to believing in the
theory of reaction built on it.

If we compare the movements of plants with those of the lower animals we
find a certain resemblance between the two. According to Jennings (H.S.
Jennings, "The Behavior of the Lower Animals". Columbia U. Press, N.Y.
1906.) a Paramoecium constantly tends to swerve towards the aboral side
of its body owing to certain peculiarities in the set and power of
its cilia. But the tendency to swim in a circle, thus produced, is
neutralised by the rotation of the creature about its longitudinal
axis. Thus the direction of the swerves IN RELATION TO THE PATH of the
organi