parts of stems are continually moving in circles or ellipses. "To learn
how the sweeps are made, one has only to mark a line of dots along the
upper side of the outstretched revolving end of such a stem, and to note
that when it has moved round a quarter of a circle, these dots will be on
one side; when half round, the dots occupy the lower side; and when the
revolution is completed, they are again on the upper side. That is, the
stem revolves by bowing itself over to one side,--is either pulled over or
pushed over, or both, by some internal force, which acts in turn all round
the stem in the direction in which it sweeps; and so the stem makes its
circuits without twisting."[1]

[Footnote 1: How Plants Behave. By Asa Gray. Ivison, Blakeman, Taylor &
Co., New York, 1872. Page 13.]

The nature of the movement is thus a successive nodding to all the points
of the compass, whence it is called by Darwin _circumnutation_. The
movement belongs to all young growing parts of plants. The great sweeps of
a twining stem, like that of the Morning-Glory, are only an increase in
the size of the circle or ellipse described.[1]

[Footnote 1: "In the course of the present volume it will be shown
that apparently every growing part of every plant is continually
circumnutating, though often on a small scale. Even the stems of seedlings
before they have broken through the ground, as well as their buried
radicles, circumnutate, as far as the pressure of the surrounding earth
permits. 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. Thus the great sweeps made by
the stems of the twining plants, and by the tendrils of other climbers,
result from a mere increase in the amplitude of the ordinary movement of