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.
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