344 SPECIAL DIRECTIONS AND 



the cells on the two sides. "We might therefore expect more prompt 

 and obvious changes of position in leaves than in stems. Familiar 

 examples of the kind are met with in the altered nocturnal position 

 of the leaves, &c. of many plants (often drooping, or folded as if in 

 repose), which Linnaeus designated by the fanciful name of 



671. The Sleep of Plants. This is well seen in the foliage of the 

 Locust and of most Leguminous plants, and in those of Oxalis, or 

 Wood-SoiTel. It is most striking in the leaflets of compound leaves. 

 The nocturnal position is various in different species, but uniform in 

 the same species, showing that the phenomenon is not mechanical. 

 Nor is it a passive state, for, instead of drooping, as do those of the 

 common Locust-tree, the leaflets are very commonly turned upwards, 

 as those of Honey-Locust, or upwards and forwards, as in the Sensi- 

 tive-Plant, contrary to the position into which they Mould fall from 

 their own weight. De Candolle found that most plants could be 

 made to acknowledge an artificial day and night, by keeping them in 

 darkness during the day, and by illuminating artificially at niglit. 

 The sensibility to light appears to reside in the petiole, and not in 

 the blade of the leaf or leaflet ; for these movements are similarly 

 executed, when nearly the whole surface of the latter is cut away. 



672. Tlie leaves of the blossom also assume various positions, 

 according to the intensity and duration of the light. Many expand 

 their blossoms in the morning and close them towards evening, 

 never to be opened again, as those of Cistus, Portulaca, and Spider- 

 wort ; while others, like tlie Crocus, close when the sun is witli- 

 drawn, but expand again the following morning. On the other hand, 

 the Evening Primrose, Silene noctiflora, &c. unfold their petals at 

 twilight, and close at sunrise. The White "VYater-Lily (Nymphasa) 

 expands in the full light of day, but uniformly closes near the mid- 

 dle of the afternoon, and is then usually withdrawn beneath the sur- 

 face of the water. The Morning- Glory opens at the dawn ; the 

 Lettuce, and most Cichoraceous plants, a few hours later, but close 

 under the noonday sun ; the Mirabilis is called Four-o'clock, because 

 opening nearly at that hour in the afternoon, and it closes the next 

 morning; and so of other species, — each having its own hour or 

 amount of light in which its blossoms open or close. Berthelot men- 

 tions an Acacia at Teneriffe, whose leaflets regularly close at sunset 

 and unfold at sunrise, while its flowers close at sunrise and unfold 

 at sunset. Although these movements, both in leaves and blossoms, 

 are undoubtedly dependent on the light, they are by no means directly 



