PERIODICAL MOVEMENTS. 861 
resume as nearly as possible their original position. In leaves, 
these periodical movements consist in the closing up of such 
organs towards the evening and their expansion in the morning. 
In the petals of flowers great differences occur in opening or 
closing at particular hours of the day ; and, by observing these 
changes in a variety of flowers, Linnzeus and others have drawn 
up what has been termed a floral clock. This periodical closing 
up of leaves and flowers has been called the sleep of plants. 
The compound leaves of certain Leguminose and Oxalidacexe 
are marked illustrations of these periodical movements, which 
are probably all indirectly dependent upon the varying con- 
ives Wile 
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Fig. 1174. Desmodium gyrans. A. Stem with leaves during the day, B. A 
similar stem with leaves asleep at night, pointing downwards. (Atter 
Darwin.) 
ditions of light to which the parts of the plant in which they 
occur are exposed. All these movements Darwin considers to 
be due to modified circumnutation. This author says: ‘In 
Lupinus the leaflets move either upwards or downwards ; and 
in some species (for instance, L. luteus), those on one side of the 
star-shaped leaf move up, and those on the opposite side move 
down ; the intermediate ones rotating on their axes; and by 
these varied movements the whole leaf forms at night a vertical 
star, instead of a horizontal one as during day. Some leaves 
and leaflets, besides moving either upwards or downwards, 
become more or less folded at night, as in Bauhinia and in 
