Nyctitropism
and nyctitropic, i.e. night-turning, may be applied both to leaves and
flowers, and will be occasionally used by us; but it would be best to
confine the term to leaves. The leaves of some few plants move either
upwards or downwards when the sun shines intensely on them, and this
movement has sometimes been called diurnal sleep; but we believe it to be
of an essentially different nature from the nocturnal movement, and it will
be briefly considered in a future chapter.
The sleep or nyctitropism of leaves is a large subject, and we think that
the most convenient plan will be first to give a brief account of the
position which leaves assume at night, and of the advantages apparently
thus gained. Afterwards the more remarkable cases will be described in
detail, with respect to cotyledons in the present chapter, and to leaves in
the next chapter. Finally, it will be shown that these movements result
from circumnutation, much modified and regulated by the alternations of day
and night, or light and darkness; but that they are also to a certain
extent inherited.
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