Although the downward movement
cannot be attributed to the weight of the cotyledons in the several cases
which were investigated, namely, in those of the Anoda, Ipomoea purpurea
and bona-nox, nor in that of I. coccinea, yet bearing in mind that
cotyledons are continually circumnutating, a slight cause might at first
have determined whether the great nocturnal movement should be upwards or
downwards. We may therefore suspect that in some aboriginal member of the
groups in question, the weight of the cotyledons first determined the
downward direction. The fact of the cotyledons of these species not sinking
down much whilst they are young and tender, seems opposed to the belief
that the greater movement when they are
[page 313]
grown older, has been acquired for the sake of protecting them from
radiation at night; but then we should remember that there are many plants,
the leaves of which sleep, whilst the cotyledons do not; and if in some
cases the leaves are protected from cold at night whilst the cotyledons are
not protected, so in other cases it may be of more importance to the
species that the nearly full-grown cotyledons should be better protected
than the young ones.
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