They often rotate at least as much as 90o on
their own axes. The leaves which arise from upright and from horizontal or
much inclined branches on the same plant, move in some few cases in a
different manner, as with Porlieria and Strephium. The whole appearance of
many plants is wonderfully changed at night, as may be seen with Oxalis,
and still more plainly with Mimosa. A bush of Acacia Farnesiana appears at
night as if covered with little dangling bits of string instead of leaves.
Excluding a few genera not seen by ourselves, about which we are in doubt,
and excluding a few others the leaflets of which rotate at night, and do
not rise or sink much, there are 37 genera in which the leaves or leaflets
rise, often moving at the same time towards the apex or towards the base of
the leaf, and 32 genera in which they sink at night.
The nyctitropic movements of leaves, leaflets, and
[page 396]
petioles are effected in two different ways; firstly, by alternately
increased growth on their opposite sides, preceded by increased turgescence
of the cells; and secondly by means of a pulvinus or aggregate of small
cells, generally destitute of chlorophyll, which become alternately more
turgescent on nearly opposite sides; and this turgescence is not followed
by growth except during the early age of the plant.
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