Leaves, when they go to sleep, move either upwards or downwards, or in the
case of the leaflets of com-
* Ch. Royer must, however, be excepted; see 'Annales des Sc. Nat.' (5th
series), Bot. vol. ix. 1868, p. 378.
[page 282]
pound leaves, forwards, that is, towards the apex of the leaf, or
backwards, that is, towards its base; or, again, they may rotate on their
own axes without moving either upwards or downwards. But in almost every
case the plane of the blade is so placed as to stand nearly or quite
vertically at night. Therefore the apex, or the base, or either lateral
edge, may be directed towards the zenith. Moreover, the upper surface of
each leaf, and more especially of each leaflet, is often brought into close
contact with that of the opposite one; and this is sometimes effected by
singularly complicated movements. This fact suggests that the upper surface
requires more protection than the lower one. For instance, the terminal
leaflet in Trifolium, after turning up at night so as to stand vertically,
often continues to bend over until the upper surface is directed downwards
whilst the lower surface is fully exposed to the sky; and an arched roof is
thus formed over the two lateral leaflets, which have their upper surfaces
pressed closely together.
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