TWINERS AND CLIMBERS. 201 



a petiole curled completely round a stick in twelve 

 hours. These petioles were left curled for twenty- 

 four hours, and the sticks were then removed, but 

 they never straightened themselves. I took a twig 

 thinner than the petiole itself, and with it lightly 

 rubbed several petioles four times, up and down ; 

 these in an hour and three-quarters became slightly 

 curled ; the curvature increased during some hours, 

 and then began to decrease, but after twenty-five 

 hours from the time of rubbing, a vestige of the 

 curvature remained. Some other petioles similarly 

 rubbed twice, that is, once up and once down, became 

 perceptibly curved in about two hours and a half. 

 They became straight again in about twelve hours." ^ 

 When the petiole embraces a twig it swells per- 

 ceptibly for two or three days, and ultimately becomes 

 twice as thick as one which has embraced nothing. 

 The same happens also in the case of other leaf- 

 climbers. A section of such a swollen petiole, when 

 examined under the microscope, exhibited an entire 

 change of structure, whereby it had become more 

 rigid and woody, simulating the structure of the 

 stem. It would seem, therefore, that this change in 

 the structure of the clasping petiole is one likely 

 to be serviceable to the plant, by giving greater 

 strength to the curved portion, and thus enabling 



' Darwin, " Movements of Climbing Plants," p. 57. 



