Cuap. IV. SPIRAL CONTRACTION. 163 
never contract spirally unless they have first seized 
hold of some object; if they catch nothing they hang 
down, remaining straight, until they wither and drop 
off: this is the case with the tendrils of Bignonia, 
which consist of modified leaves, and with those of 
three genera of the Vitacese, which are modified flower- 
peduncles. But in the great majority of cases, tendrils 
which have never come in contact with any object, 
after a time contract spirally. All these facts taken 
together, show that the act of clasping a support and 
the spiral contraction of the whole length of the 
tendril, are phenomena not necessarily connected. 
The spiral contraction which ensues after a tendril 
has caught a support is of high service to the plant; 
hence its almost universal occurrence with species 
belonging to widely different orders. When a shoot 
is inclined and its tendril has caught an object above, 
the spiral contraction drags up the shoot. When the 
shoot is upright, the growth of the stem, after the 
tendrils have seized some object above, would leave it 
slack, were it not for the spiral contraction which 
draws up the stem as it increases in length. Thus 
there is no waste of growth, and the stretched stem 
ascends by the shortest course. When a terminal 
branchlet of the tendril of Coba#a catches a stick, we 
have seen how well the spiral contraction successively 
brings the other branchlets, one after the other, into 
contact with the stick, until the whole tendril grasps 
it in an inextricable knot. When a tendril has caught 
a yielding object, this is sometimes enveloped and 
