, and another in 6
m. The former became bent at right angles in 15 min., and became
straight again in between 5 hrs. and 6 hrs. A loop of thread
weighing 0.125th of a grain caused another petiole to curve.
Tropaeolum pentaphyllum.--This species has not the power of spirally
twining, which seems due, not so much to a want of flexibility in the
stem, as to continual interference from the clasping petioles. An
upper internode made three revolutions, following the sun, at an
average rate of 1 hr. 46 m. The main purpose of the revolving
movement in all the species of Tropaeolum manifestly is to bring the
petioles into contact with some supporting object. The petiole of a
young leaf, after a slight rub, became curved in 6 m.; another, on a
cold day, in 20 m., and others in from 8 m. to 10 m. Their curvature
usually increased greatly in from 15 m. to 20 m., and they became
straight again in between 5 hrs. and 6 hrs., but on one occasion in 3
hrs. When a petiole has fairly clasped a stick, it is not able, on
the removal of the stick, to straighten itself. The free upper part
of one, the base of which had already clasped a stick, still retained
the power of movement.
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