Cuar. IL. TROPAZOLUM. 63 
to move perceptibly in 5m, and another in 6m. 
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 }th of a grain caused another 
petiole to curve. 
Tropxolum pentaphyllum.—tThis 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 inter- 
node 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 Tro- 
pxolum 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 6m.; another, 
on a cold day, in 20m., and others in from 8 m. 
to10m. Their curvature usually increased greatly in 
from 15m. 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, itis 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. A 
loop of thread weighing 3th of a grain caused a petiole 
to curve; but the stimulus was not sufficient, the loop 
remaining suspended, to cause a permanent flexure. If 
a much heavier loop be placed in the angle between 
the petiole and the stem, it produces no effect ; whereas 
we have seen with Clematis montana that the angle 
between the stem and petiole is sensitive. 
