With Arachis, all four leaflets form together
during the night a single vertical packet; and to the effect this the two
anterior leaflets have to move upwards and the two posterior ones forwards,
besides all twisting on their own axes. In the genus Sida the leaves of
some species move at night through an angle of 90o upwards, and of others
[page 395]
through the same angle downwards. We have seen a similar difference in the
nyctitropic movements of the cotyledons in the genus Oxalis. In Lupinus,
again, the leaflets move either upwards or downwards; and in some species,
for instance L. luteus, those on one side of the star-shaped leaf move up,
and those on the opposite side move down; the intermediate ones rotating on
their axes; and by these varied movements, the whole leaf forms at night a
vertical star instead of a horizontal one, as during the day. Some leaves
and leaflets, besides moving either upwards or downwards, become more or
less folded at night, as in Bauhinia and in some species of Oxalis. The
positions, indeed, which leaves occupy when asleep are almost infinitely
diversified; they may point either vertically upwards or downwards, or, in
the case of leaflets, towards the apex or towards the base of the leaf, or
in any intermediate position.
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