8 MK. DARWIN ON CLIMBING TLANTP. 



tudinal contracting surface slowly creep round the shoot, desert- 

 ing by slow degrees ihe southern side and encroaching on the 

 eastern side, and so round by the north, by the west, again to the 

 south ; in this case the shoot would remain always bowed with 

 the painted line appearing on the convex, on the lateral, and con- 

 cave surfaces, and with the point of the shoot successively di- 

 rected to all points of the compass. In fact, we should then have 

 the exact kind of movement seen in the revolving shoots of 1 wi- 

 ning plants. I have spoken in the illustration, for brevity's sake, 

 of the cells along each face successively contracting; of course 

 iurgcscencc of the cells on the opposite face, or both forces com- 

 bined, would do equally well. 



It must not be supposed that the revolving movement of twi- 

 ning plants is as regular as that given in this illustration ; in 

 very many cases the tip describes an ellipse, even a very narrow- 

 ellipse. To recur once again to our illustration, if wo suppose 

 the southern and then the northern face of the sapling to con- 

 tract, the summit woxild describe a simple arc ; if the contraction 

 first travelled a very little to the eastern face, and during the 

 return a very little to the western face, a narrow ellipse would be 

 described; and the sapling would become siraight as it passed to 

 and fro by the central point. A complete straightening of the 

 shoot may often be observed in revolving plants ; but the weight 

 of the shoot apparently interferes with ihe regularity of the 

 movement, and with the place of straitening. The movement is 

 often (in appearance at least) as if the southern, eastern, and 

 northern faces had contracted, but not the western face ; so that 

 a semicircle is described, and the shoot becomes straight and up- 

 right in one part of its course. 



When a revolving shoot consists of several internodes, the 

 several lower ones bend together at the same rate, but the one 

 or two terminal internodes bend at a slower rate ; hence, though 

 at times all the internodes may be bowed in the same line, at 

 other times the shoot is rendered slightly serpentine, as I have 

 often observed. The rate of revolution of the whole shoot, if 

 judged by the movement of the extreme tip, is thus at times 

 accelerated and retarded. One other point must be noticed. 

 Authors have observed that the end of the shoot in many twining 

 plants is completely hooked ; this is very general, for instance, 

 with the Asclepiadacca\ The hooked tip, in all the cases which 

 I observed, viz. in Ceropegia, Splucrosteina, Clerodendron, Wis- 

 taria, Stephania, AJcelia, and SipJiomeris, has exactly the same kind 

 of movement as the other revolving internodes ; for a line painted 



