The figures thus traced were
therefore angular, but if dots had been made every one or two minutes, the
lines would have been more curvilinear."--The Power of Movement in Plants,
p. 6.]
The use of the glass filament is simply to increase the size of the circle
described, and thus make visible the movements of the stem. All young
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.
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