THE SLEEP OF PLANTS. 343 



ing together of their leaflets, as if in repose, when the stimulus 

 of light is removed. This is well seen in the foliage of the 

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

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

 leaves. Their 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 if by their own weight, the leaflets are more commonly turned 

 upwards or forwards, contrary to the position into which they 

 would 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 

 them with candles at night. 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. 



655. The 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 and of many Portula- 

 caceous plants ; while others, like the Crocus, close when the sun 

 is withdrawn, but expand again the following morning.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 Water- 

 Lily (Nympha3a) expands in the full light of day, but uniformly 

 closes near the middle of the afternoon, and is then usually with- 

 drawn beneath the surface of the water. The Morning Glory 

 opens at the dawn ; the Lettuce and most Cichoraceous plants, a 

 few hours later; the Mirabilis, or Four-o'clock plant, nearly at 

 that hour in the afternoon, &c. Berthelot mentions an Acacia at 

 TenerifFe, whose leaflets regularly close at sunset and unfold at 

 sunrise, while its flowers close at sunrise and unfold at sunset.* 



* The odors of flowers, also, are sometimes given off continually, as in the 

 Orange and the Violet, or else they nearly lose their fragrance during the heat 

 of mid-day, as in most cases ; while others, such as Pelargonium triste, Hes- 

 peris tristis, and most dingy flowers, which are almost scentless during the 

 day, exhale a powerful fragrance at night. The night-flowering Cereus gran- 

 diflorus emits its powerful fragrance at intervals ; sudden emanations of odor 

 being given off about every quarter of an hour, during the brief period of the 

 expansion of the flower. 



