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.

[Footnote 1: See Physiological Botany. By Geo. L. Goodale. Ivison & Co.,
New York, 1885. Page 406.]

[Footnote 2: The following experiment exhibits the phenomenon of tension
very strikingly. "From a long and thrifty young internode of grapevine
cut a piece that shall measure exactly one hundred units, for instance,
millimeters. From this section, which measures exactly one hundred
millimeters, carefully separate the epidermal structures in strips, and
place the strips at once under an inverted glass to prevent drying;
next, separate the pith in a single unbroken piece wholly freed from the
ligneous tissue. Finally, remeasure the isolated portions, and compare
with the original measure of the internode. There will be found an
appreciable shortening of the epidermal tissues and a marked increase in
length of the pith."--Physiological Botany, p. 391.]

The movements of the Sensitive Plant are always very interesting to
pupils, and it is said not to be difficult to raise the plants in the
schoolroom. The whole subject, indeed, is one of the most fascinating
that can be found, and its literature is available, both for students and
teachers. Darwin's essay on "Climbing Plants," and his later work on the
"Power of Movement in Plants," Dr. Gray's "How Plants Behave," and the
chapter on "Movements" in the "Physiological Botany," will offer a wide