100 Mtt. DARWIN ON CLIMBING PLANTS. 



several plants a single touch, so slight as only just to move the 

 highly flexible tendril, is enough to induce curvature. Passijlora 

 gracilis has the most sensitive tendrils which 1 bare seen: a hit 

 of platina wire J^th of a grain in weight, gently placed on the 

 concave point, caused two tendrils to Income hooked, as did (and 

 this perhaps is a better proof of sensitiveness) a loop of soft, thin 

 cotton thread weighing ^nd of a grain, or about two milli- 

 grammes. With the tendrils of several other plants, loops weigh- 

 ing -}V tu °f a g 1 ':' 1 " sufficed- The point of the tendril of the Pas- 

 sijlora gracilis distinctly began to move in 2o seconds after a 

 touch. Asa Gray saw movement in the tendrils of the Cucurbi- 

 taceous genus, Sicyos, in 30 seconds. The tendrils of some other 

 plauts, when lightly rubbed, move in a few minutes ; in the 

 Dicentra in half-an-hour ; in the SmUase in an hour and a quarter 

 or a half; and in the Ampclopsis still more slowly. The curling 

 movement consequent on a single touch continues to increase for 

 a considerable time, then ceases ; after a few hours the tendril 

 uncurls itself, and is again ready for action. When very light 

 weights were suspended on tendrils of several plants and caused 

 them to curve, these seemed to become accustomed to so slight a 

 stimulus, and straightened themselves, as if the loops had been 

 removed. It makes no difference, as far as 1 have seen, what sort 

 of object a tendril touches, with the remarkable exception of drops 

 of water in the case of the extremely sensitive tendrils of Passijlora 

 gracilis and of the Ju-liinocystis ; hence we are led to infer that 

 they have become habituated to showers of rain. As I made no 

 observations with this view on other tendrils, 1 cannot say whether 

 there are more cases of this adaptation. Moreover adjoining ten- 

 drils rarely catch each other, as we have seen with the Echinocys lis 

 and Passijlora, though I have seen this occur with the Bryony. 



Tendrils of which the extremities are slightly curved or bowed 

 are sensitive only on the concave surface ; other tendrils, such as 

 those of the Coba?a (though furnished with minute horny hooks) 

 and those of Cissus discolor, are sensitive on all sides. Hence 

 the tendril of this latter plant, when stimulated by a touch of 

 equal force on opposite sides, docs not bend. In the tendril of 

 the Mutisia the inferior and lateral surfaces are sensitive, but not 

 the upper surface. With branched tendrils, the several branches 

 all act alike; but in the Hanburya the lateral spur-like branch 

 does not acquire (for a reason which has been explained) its sen- 

 sitiveness nearly so soon as the main branch. The lower or basal 

 part of many tendrils is either not at all sensitive or sensitive only 



