[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 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 slit vertically for an inch or two, the two halves will spring back abruptly.  This is because the outer tissues of the stem are stretched, and spring back like india-rubber when released.  If two stalks twining in opposite directions be slit as above described, the side of the stem towards which each stalk is bent will spring back more than the other, showing the tension to be greater on that side.  A familiar illustration of this tension will be found in the Dandelion curls of our childhood.