Here we have the unusual case of one of the
leaflets not standing vertically, or almost vertically, at night.
Considering that leaves in assuming their nyctitropic positions often move
through an angle of 90o; that the movement is rapid in the evening; that in
some cases, as we shall see in the next chapter, it is extraordinarily
complicated; that with certain seedlings, old enough to bear true leaves,
the cotyledons move vertically upwards at night, whilst at the same time
the leaflets move vertically downwards; and that in the same genus the
leaves or cotyledons of some species move upwards, whilst those of other
species move downwards;--from these and other such facts, it is hardly
possible to doubt that plants must derive some
[page 283]
great advantage from such remarkable powers of movement.
The nyctitropic movements of leaves and cotyledons are effected in two
ways,* firstly, by means of pulvini which become, as Pfeffer has shown,
alternately more turgescent on opposite sides; and secondly, by increased
growth along one side of the petiole or midrib, and then on the opposite
side, as was first proved by Batalin.
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