Fifthly, tendrils, whatever their homological nature may be, and the
petioles or tips of the leaves of leaf-climbers, and apparently
certain roots, all have the power of movement when touched, and bend
quickly towards the touched side. Extremely slight pressure often
suffices. If the pressure be not permanent, the part in question
straightens itself and is again ready to bend on being touched.
Sixthly, and lastly, tendrils, soon after clasping a support, but not
after a mere temporary curvature, contract spirally. If they have
not come into contact with any object, they ultimately contract
spirally, after ceasing to revolve; but in this case the movement is
useless, and occurs only after a considerable lapse of time.
With respect to the means by which these various movements are
effected, there can be little doubt from the researches of Sachs and
H. de Vries, that they are due to unequal growth; but from the
reasons already assigned, I cannot believe that this explanation
applies to the rapid movements from a delicate touch.
Finally, climbing plants are sufficiently numerous to form a
conspicuous feature in the vegetable kingdom, more especially in
tropical forests.
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