This, together with the dependent position of the leaflets,
makes the whole plant wonderfully compact at night. In the two foregoing
figures, copied from photographs, the same plant is represented awake and
asleep (Fig. 155), and we see how different is its appearance.
Cassia mimosoides.--At night the numerous leaflets on each leaf rotate on
their axes, and their tips move towards the apex of the leaf; they thus
become imbricated with their lower surfaces directed upwards, and with
their midribs almost parallel to the petiole. Consequently, this species
differs from all the others seen by us, with the exception of the following
one, in the leaflets not sinking down at night. A petiole, the movement of
which was measured, rose 8o at night.
Cassia Barclayana.--The leaflets of this Australian species are numerous,
very narrow, and almost linear. At night they rise up a little, and also
move towards the apex of the leaf. For instance, two opposite leaflets
which diverged from one another during the day at an angle of 104o,
diverted at night only 72o; so that each had risen 16o above its diurnal
position.
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