The petiole sinks a little. In
Gleditschia, if we understand correctly Duchartre's description, and in
Poin-
[page 369]
ciana Gilliesii (both belonging to Tribe 13), the leaves behave in the same
manner.
Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B, branch
with leaves asleep, reduced to two-thirds of natural scale.
Cassia (Tribe 14).--The nyctitropic movements of the leaves in many species
in this genus are closely alike, and are highly complex. They were first
briefly described by Linnaeus, and since by Duchartre. Our observations
were made chiefly on C. floribunda* and corymbosa, but several other
species were casually observed. The horizontally extended leaflets sink
down vertically at night; but not simply, as in so many other genera, for
each leaflet rotates on its own axis, so that its lower surface faces
outwards. The upper surfaces of the opposite leaflets are thus brought into
contact with one another beneath the petiole, and are well protected (Fig.
154). The rotation and other movements are effected by means of a
well-developed pulvinus at the base of each leaflet, as could be plainly
seen when a straight narrow black line had been painted along it during the
day.
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