HABITS OF CLIMBING PLANTS. 169 



is, the petioles or their divisions are sensitive to the contact of 

 an extraneous body, contracting on the side touched so as to 

 curve or coil around it. That the footstalk is directly sensi- 

 tive to the touch, just as tendrils are, Mr. Darwin proved by 

 lightly rubbing them with a twig for a few times, when in the 

 course of some hours it bends to the rubbed side, afterwards 

 becoming straight again ; or by leaving the body in contact it 

 is permanently clasped by the footstalk. So sensitive are some 

 footstalks that " a loop of thread weighing a quarter of a grain 

 caused them to bend ; a loop weighing one-eighth of a grain 

 sometimes acted, and sometimes not." In one instance, in 

 Clematis Flammula, even the sixteenth part of a grain caused 

 a petiole to bend through nearly 90 degrees. With rare 

 exceptions only the young petioles are sensitive. Take the 

 cultivated Clematis Viticella for an illustration of the mode in 

 which the leaves do the work of climbing. 



"The leaves are of large size. There are three pairs of 

 lateral leaflets and a terminal one, all borne by rather long 

 petioles. The main petiole bends a little, angularly, down- 

 ward at each point where a pair of leaflets arises, and the 

 petiole of the terminal leaflet is bent downward at right angles ; 

 hence the whole petiole, with its rectangularly bent extremity, 

 acts as a hook. This, with the lateral petioles directed a little 

 upward, forms an excellent grappling apparatus by which the 

 leaves readily become entangled with surrounding objects. If 

 they catch nothing, the whole petiole ultimately grows straight. 

 Both the medial and lateral petioles are sensitive ; and the 

 three branches, into which the basi-lateral petioles are gener- 

 ally subdivided, likewise are sensitive. The basal portion of 

 the main petiole, between the stem and the first pair of leaf- 

 lets, is less sensitive than the remainder, but it will clasp a 

 stick when in contact. On the other hand, the inferior sur- 

 face of the rectangularly bent terminal portion (carrying the 

 terminal leaflet), which forms the inner side of the end of the 

 hook, is the most sensitive part ; and this portion is manifestly 

 best adapted to catch distant supports. To show the differ- 

 ence in sensibility, I gently placed loops of string of the same 

 weight (in one instance weighing .82 of a grain) on the sev- 



