--The leaflets sink vertically down at night.
Phaseolus vulgaris (Tribe 8).--The leaflets likewise sink vertically down
at night. In the greenhouse the petiole of a young leaf rose 16o, and that
of an older leaf 10o at night. With plants growing out of doors the
leaflets apparently do not sleep until somewhat late in the season, for on
the nights of July 11th and 12th none of them were asleep; whereas on the
night of August 15th the same plants had most of their leaflets vertically
dependent and asleep. With Ph. caracalla and Hernandesii, the primary
unifoliate leaves and the leaflets of the secondary trifoliate leaves sink
vertically down at night. This holds good with the secondary trifoliate
leaves of Ph. Roxburghii, but it is remarkable that the primary unifoliate
leaves which are much elongated, rise at night from about 20o to about 60o
above the horizon. With older seedlings, however, having the secondary
leaves just developed, the primary leaves stand in the middle of the day
horizontally, or are deflected a little beneath the horizon. In one such
case the primary leaves rose from 26o beneath the horizon at noon, to 20o
above it at 10 P.
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