Page:Darwin - On the movements and habits of climbing plants.djvu/34

 were left curled for 24 h., and then the sticks were removed; but they never straightened themselves. I took a twig, thinner than the petiole itself, and lightly rubbed with it several petioles four times up and down; these in 1 h. 45 m. became slightly curled; the curvature increased during some hours and then began to decrease, but after 25 h. from the time of rubbing a vestige of the curvature remained. Some other petioles similarly rubbed once up and down became perceptibly curved in about 2 h. 30 m., a terminal sub-petiole moving more than a lateral sub-petiole; they became quite straight again in between 12 h. and 14 h. Lastly, a length of about one-eighth of an inch of a sub-petiole, lightly rubbed with the same twig only once down, became slightly curved in 3 h., and remained so during 11 h., but the next morning was quite straight.

The following observations are more precise. After finding that heavier pieces of string and thread acted, I placed a loop of string, weighing 1.04 gr., on a terminal petiole: in 6 h. 40 m. a curvature could be seen; in 24 h. the petiole formed an open ring round the string; in 48 h. the ring had almost closed on the string, and in 72 h. it had firmly seized the fine twine so that it required some force to withdraw it. A loop weighing .52 of a grain caused a lateral sub-petiole just perceptibly to curve in 14 h., but after 24 h. it had moved through ninety degrees. These observations were made during the summer: the following were made in the spring, when the petioles are apparently more sensitive:—A loop of thread, weighing one-eighth of a grain, produced no effect on the lateral sub-petioles, but placed on a terminal one caused, after 24 h., a moderate curvature in it; the curvature, though the loop remained suspended, was after 48 h. diminished, but never disappeared, showing that the petiole had become partially accustomed to the insufficient stimulus. This experiment was twice repeated with nearly similar results. Lastly, a loop of thread, weighing only one-sixteenth of a grain (nearly equal to four milligrammes), was twice gently placed by a forceps on a terminal sub-petiole (the plant being, of course, in a still and closed room), and this weight certainly caused a flexure, which very slowly increased until the petiole had moved through nearly ninety degrees: beyond this it did not move; nor did the petiole, the loop remaining suspended, ever become perfectly straight again.

When we consider, on the one hand, the thickness and stiffness of the petioles, and, on the other hand, the thinness and softness of fine cotton thread, and what an extremely small weight