LEAF-CLI.MBEHS. 33 



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 de- 

 crease, 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 2h. 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, 1 placed a loop of 

 string, weighing 101 gr., on a terminal petiole : in 6 h. 40 m. a cur- 

 vature could be seen ; in 21 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 2 1 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 re- 

 mained suspended, was after 48 h. diminished, but never dis- 

 appeared, showing that the petiole had become partially accus- 

 tomed 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 milli- 

 grammes), 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 : bevond 

 this it did not move; nor did the petiole, the loop remaining sus- 

 pended, 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 one- 



