Leaves and cotyledons assume their nocturnal position by two means,
by the aid of pulvini and without such aid. In the former case the movement
continues as long as the leaf or cotyledon remains in full health; whilst
in the latter case it continues only whilst the part is growing. Cotyledons
appear to sleep in a larger proportional number of species than do leaves.
In some species, the leaves sleep and not the cotyledons; in others, the
cotyledons and not the leaves; or both may sleep, and yet assume widely
different positions at night.
Although the nyctitropic movements of leaves and cotyledons are wonderfully
diversified, and sometimes differ much in the species of the same genus,
yet the blade is always placed in such a position at night, that its upper
surface is exposed as little as possible to full radiation. We cannot doubt
that this is the object gained by these movements; and it has been proved
that leaves exposed to a clear sky, with their blades compelled to remain
horizontal, suffered much more from the cold than others which were allowed
to assume
[page 561]
their proper vertical position.
Pages:
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933