The object gained is the protection of the upper
surfaces of the leaves from radiation at night, often combined with the
mutual protection of the several parts by their close approximation. In
such cases as those of the leaflets of Cassia--of the terminal leaflets of
Melilotus--of all the leaflets of Arachis, Marsilea, etc.--we have ordinary
circumnutation modified to the extreme extent known to us in any of the
several great classes of modified circumnutation. On this view of the
origin of nyctitropism we can understand how it is that a few plants,
widely distributed throughout the Vascular series, have been able to
acquire the habit of placing the blades of their leaves vertically at
night, that is, of sleeping,--a fact otherwise inexplicable.
The leaves of some plants move during the day in a manner, which has
improperly been called diurnal sleep; for when the sun shines brightly on
them, they direct their edges towards it. To such cases we shall recur in
the following chapter on Heliotropism. It has been shown that the leaflets
of one form of Porlieria hygrometrica keep closed during the day, as long
as the plant is scantily supplied with water, in the same manner as when
asleep; and this apparently serves to check evaporation.
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