1863.] INSECTIVOROUS PLANTS. 313 



you could give or lend me, or I could buy, with tendrils, re- 

 markable in any way for development, for odd or peculiar 

 structure, or even for an odd place in natural arrangement. I 

 have seen or can see Cucurbitaceae, Passion-flower, Virginian- 

 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 before 

 and correctly, as further observation shows) ; for instance, I find 

 a slight pinch between the thumb and finger at the end of the 

 tendril of the Cucurbitaceae 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 movement, 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 independent of 

 the movement of the upper internodes, but both work har- 

 moniously 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 



