LIFE OF PLANTS. 



241 



367. 



The periodical change in the direction of leaves, whicH 

 has been called the Sleep of Plants, is undeniably connected 

 with this stimulating operation of light. It is established, that 

 during the clear light of the sun, the leaves become erect, and 

 move their upper surface to the light, whilst, on the contrary, 

 during the absence of light, they either hang downwards, and 

 turn to the horizon, or they take an upright position, so that 

 the under surface of the leaves is turned more outward. On 

 account of this particular position of what have been called 

 Sleeping Plants, we cannot properly ascribe this direction to* 

 sleep, because the leaves do sometimes even raise themselves 

 during this state with greater energy, and press upon the 

 stem or leaf-stalk, for the purpose of turning their lower sur- 

 face outwards. This change is much rather, therefore, the 

 consequence of the contest we have already mentioned, be- 

 tween the activity of the plant, and the great activity of na- 

 ture. This change is the more evident, and the sleep of 

 leaves the more striking, the finer and more compounded 

 the organization of the leaves is. We hence most frequently 

 observe it in the pinnated leaves of leguminous plants, al- 

 though also in some others, as in Atriplex. 



That an internal and self-dependent activity is to be taken 

 into account in this sleep of plants, is plain from the fact that 

 this sleep does not equally follow from a short withdrawing 

 of the light, but only from its complete and long continued 

 removal; as also from this other circumstance, that leaves 

 fall asleep or awake at fixed hours, whether the sky be serene 

 or troubled, exactly as happens with regard to animals. Other 

 stimuli, too, and especially heat, have a great influence upon 

 this phenomenon, because, in the cold, leaves awaken later, and 

 fall more easily asleep, notwithstanding the influence of light, 



368. 



Blossoms also experience a similar change from the in- 

 fluence of light. Most of them unfold themselves in the 

 morning at stated hours, and again close in the evening, 

 {Bhimeii'Uhr). A considerable number, especially the Oxa-= 



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