Chap. I. TWINING PLANTS. 13 



our illustration, if we suppose only the northern and 

 southern surfaces of the sapling alternately to grow 

 rapidly, the summit would describe a simple arc ; if 

 the growth first travelled a very little to the western 

 face, and during the return a very little to the eastern 

 face, a narrow ellipse would be described ; and the 

 sapling would be straight as it passed to and fro 

 through the intermediate space ; and a complete 

 straightening of the shoot may often be observed in 

 revolving plants. The movement is frequently such 

 that three of the sides of the shoot seem to be growing 

 in due order more rapidly than the remaining side ; so 

 that a semi-circle instead of a circle is described, the 

 shoot becoming straight and upright during half of its 

 course. 



When a revolving shoot consists of several inter- 

 nodes, the lower ones bend together at the same rate, 

 but one or two of the terminal ones bend at a slower 

 rate ; hence, though at times all the internodes are 

 in the same direction, at other times the shoot is 

 rendered slightly serpentine. The rate of revolution 

 of the whole shoot, if judged by the movement of the 

 extreme tip, is thus at times accelerated or retarded. 

 One other point must be noticed. Authors have ob- 

 served that the end of the shoot in many twining plants 

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

 with the Asclepiadacese. The hooked tip, in all the 

 cases observed by me, viz. in Ceropegia, Sphserostema, 

 Clerodendron, Wistaria, Stephania, Akebia, and Sipho- 

 meris, has exactly the same kind of movement as the 



