The leaflets borne by this part are greatly reduced
in size, being, on an average, about the tenth of an inch in length
and very narrow; one small leaflet measured one-twelfth of an inch in
length and one-seventy-fifth in breadth (2.116 mm. and 0.339 mm.), so
that it was almost microscopically minute. All the reduced leaflets
have branching nerves, and terminate in little spines, like those of
the fully developed leaflets. Every gradation could be traced, until
we come to branchlets (as a and d in the figure) which show no
vestige of a lamina or blade. Occasionally all the terminal
branchlets of the petiole are in this condition, and we then have a
true tendril.
The several terminal branches of the petiole bearing the much reduced
leaflets (a, b, c, d) are highly sensitive, for a loop of thread
weighing only the one-sixteenth of a grain (4.05 mg.) caused them to
become greatly curved in under 4 hrs. When the loop was removed, the
petioles straightened themselves in about the same time. The petiole
(e) was rather less sensitive; and in another specimen, in which the
corresponding petiole bore rather larger leaflets, a loop of thread
weighing one-eighth of a grain did not cause curvature until 18 hrs.
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