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. [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 field for study and experiment. 3. _Structure of Stems_.--Let the pupils collect a series of branches of some common tree or shrub, from the youngest twig up to as large a branch as they can cut, and describe them. Poplar, Elm, Oak, Lilac, etc., will be found excellent for the purpose.