58 LEAF-CLIMBERS. Chap. II. 



length of about one-eighth of an inch of a sub-petiole, 

 was lightly rubbed with the same twig only once ; it 

 became slightly curved in 3 hrs., remaining so during 

 11 hrs., but by the next morning was quite straight. 



The following observations are more precise. After 

 trying heavier pieces of string and thread, I placed a 

 looj} of fine string, weighing 1*04 gr. (674 mg.) on a 

 terminal sub-petiole : in 6 hrs. 40 m. a curvature could 

 be seen ; in 24 hrs. the petiole formed an open ring round 

 the string ; in 48 hrs. the ring had almost closed on the 

 string, and in 72 hrs. seized it so firmly, that some 

 force was necessary for its withdrawal. A loop weighing 

 52 of a grain (33*7 mg.) caused in 14 hrs. a lateral sub- 

 petiole just perceptibly to curve, and in 24 hrs. it 

 moved through ninety degrees. These observations were 

 made during the summer : the following were made 

 in the spring, when the petioles apparently are more 

 sensitive : A loop of thread, weighing one-eighth of a 

 grain (8*01 mg.), produced no effect on the lateral sub- 

 petioles, but placed on a terminal one, caused it, after 

 24 hrs., to curve moderately ; the curvature, though the 

 loop remained suspended, was after 48 hrs. diminished, 

 but never disappeared ; showing that the petiole had 

 become partially accustomed to the insufficient stimulus. 

 This experiment was twice repeated with nearly the 

 same result. Lastly, a loop of thread, weighing only 

 one-sixteenth of a grain (4*05 mg.) 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 



