Some members of the
clan also bear blind flowers, which have been described in the
account of the white wood-sorrel given above. Even the
rudimentary leaves of the seedlings "go to sleep" at evening, and
during the day are in constant movement up and down. The stems,
too, are restless; and as for the mature leaves, every child
knows how they droop their three leaflets back to back against
the stem at evening, elevating them to the perfect horizontal
again by day. Extreme sensitiveness to light has been thought to
be the true explanation of so much activity, and yet this is not
a satisfactory theory in many cases. It is certain that drooping
leaves suffer far less from frost than those whose upper surfaces
are flatly exposed to the zenith. This view that the sleep of
leaves saves them from being chilled at night by radiation is
Darwin's own, supported by innumerable experiments; and probably
it would have been advanced by Linnaeus, too, since so many of
his observations in "Somnus Plantarum" verify the theory, had the
principle of radiation been discovered in his day.
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