Thirdly, climbing plants, like other plants, bend toward the light by a movement closely analogous to that incurvation which causes them to revolve. This similarity in the nature of the movement was well seen when plants were kept in a room, and their first movements in the morning toward the light and their subsequent revolving movements were traced on a bell glass. The movement of a revolving shoot, and in some cases of a tendril, is retarded or accelerated in traveling from or to the light. In a few instances tendrils bend in a conspicuous manner toward the dark. Many authors speak as if the movement of a plant toward the light was as directly the result of the evaporation or of the oxygenation of the sap in the stem, as the elongation of a bar of iron from an increase in its temperature. But, seeing that tendrils are either attracted to or repelled by the light, it is more probable that their movements are only guided and stimulated by its action in the same manner as they are guided by the force of attraction toward the centre of gravity. Fourthly, we have in stems, petioles, flower-peduncles and tendrils the spontaneous revolving movement which depends on no outward stimulus, but is contingent on the youth of the part and on its vigorous health, which again, of course, depends on proper temperature and the other conditions of life. This is, perhaps, the most interesting of all the movements of climbing plants because it is continuous. Very many other plants exhibit spontaneous movements, but they generally occur only once during the life of a plant, as in the movements of the stamens and pistils, etc., or at intervals of time, as in the so-called sleep of plants. Fifthly, we have in the tendrils, whatever their homological nature may be, in the petioles and tips of the leaves of leaf-climbers, in the stem in one case and apparently in the aerial roots of the vanilla, movements--often rapid movements--from contact with any body. Extremely slight pressure suffices to cause the movement. These several organs, after bending from a touch, become straight again, and again bend when touched. Sixthly, and lastly, most tendrils, soon after clasping a support, but not after a mere temporary curvature, contract spirally. The stimulus from the act of clasping some object seems to travel slowly down the whole length of the tendril. Many tendrils, moreover, ultimately contract spontaneously even if they have caught no object; but this latter useless movement occurs only after a considerable lapse of time.