Cuap. 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 Asclepiadacee. The hooked tip, in all the 
cases observed by me, viz. in Ceropegia, Sphzrostema, 
Clerodendron, Wistaria, Stephania, Akebia, and Sipho- 
merits, has exactly the same kind of movement as the 
