486 CLIMBING AND INSECTIVOROUS PLANTS. [1863, 



creeper, Cissus discolor, Common-pea and Everlasting-pea. It 

 is really curious the diversification of irritability (I do not 

 mean the spontaneous movement, about which I wrote be- 

 fore and correctly, as further observation shows) : for in- 

 stance, I find a slight pinch between the thumb and finger at 

 the end of the tendril of the Cucurbitacese* causes prompt 

 movement, but a pinch excites no movement in Cissus. The 

 cause is that one side alone (the concave) is irritable in the 

 former ; whereas both sides are irritable in Cissus, so if you 

 excite at the same time both opposite sides there is no move- 

 ment, but by touching with a pencil the two branches of the 

 tendril, in any part whatever, you cause movement towards 

 that point ; so that I can mould, by a mere touch, the two 

 branches into any shape I like. . . 



C. Darwin to Asa Gray. 



Down, August 4 [1863]. 



My present hobby-horse I owe to you, viz. the tendrils : 

 their irritability is beautiful, as beautiful in all its modifica- 

 tions as anything in Orchids. About the spontaneous move- 

 ment (independent of touch) of the tendrils and upper inter- 

 nodes, I am rather taken aback by your saying, "is it not 

 well known ? " I can find nothing in any book which I have. 

 . . . The spontaneous movement of the tendrils is independ- 

 ent of the movement of the upper internodes, but both work 

 harmoniously together in sweeping a circle for the tendrils to 

 grasp a stick. So with all climbing plants (without tendrils) 

 as yet examined, the upper internodes go on night and day 

 sweeping a circle in one fixed direction. It is surprising to 

 watch the Apocyneae with shoots 18 inches long (beyond the 

 supporting stick), steadily searching for something to climb 

 up. When the shoot meets a stick, the motion at that point 

 is arrested, but in the upper part is continued ; so that the 

 climbing of all' plants yet examined is the simple result of the 

 spontaneous circulatory movement of the upper internodes. 

 Pray tell me whether anything has been published on this 



