circumnutation."--The Power of Movement in Plants, p. 3.]

When a young stem of a Morning-Glory, thus revolving, comes in contact
with a support, it will twist around it, unless the surface is too smooth
to present any resistance to the movement of the plant. Try to make
it twine up a glass rod. It will slip up the rod and fall off. The
Morning-Glory and most twiners move around from left to right like the
hands of a clock, but a few turn from right to left.

While this subject is under consideration, the tendrils of the Pea and
Bean and the twining petioles of the Nasturtium will be interesting for
comparison. The movements can be made visible by the same method as was
used for the stem of the Morning-Glory. Tendrils and leaf petioles are
often sensitive to the touch. If a young leaf stalk of Clematis be rubbed
for a few moments, especially on the under side, it will be found in a day
or two to be turned inward, and the tendrils of the Cucumber vine will
coil in a few minutes after being thus irritated.[1] The movements of
tendrils are charmingly described in the chapter entitled "How Plants
Climb," in the little treatise by Dr. Gray, already mentioned.

[Footnote 1: Reader in Botany. X. Climbing Plants.]

The so-called "sleep of plants" is another similar movement. The Oxalis is
a good example. The leaves droop and close together at night, protecting
them from being chilled by too great radiation.

The cause of these movements is believed to lie in changes of tension
preceding growth in the tissues of the stem.[1] Every stem is in a state
of constant tension. Naudin has thus expressed it, "the interior of every
stem is too large for its Jacket."[2] If a leaf-stalk of Nasturtium be