If this does not happen, they retain their hooked shape for a
considerable time, and then bending upwards reassume their original
upturned position, which is preserved ever afterwards. The petioles
which have clasped any object soon become much thickened and
strengthened, as may be seen in the drawing.
Clematis montana.--The long, thin petioles of the leaves, whilst
young, are sensitive, and when lightly rubbed bend to the rubbed
side, subsequently becoming straight. They are far more sensitive
than the petioles of C. glandulosa; for a loop of thread weighing a
quarter of a grain (16.2 mg.) caused them to bend; a loop weighing
only one-eighth of a grain (8.1 mg.) sometimes acted and sometimes
did not act. The sensitiveness extends from the blade of the leaf to
the stem. I may here state that I ascertained in all cases the
weights of the string and thread used by carefully weighing 50 inches
in a chemical balance, and then cutting off measured lengths. The
main petiole carries three leaflets; but their short, sub-petioles
are not sensitive. A young, inclined shoot (the plant being in the
greenhouse) made a large circle opposed to the course of the sun in 4
hrs.
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