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 Cobaea 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.
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