114 



HISTOLOGY AND PHYSIOLOGY. 



a way ttat their apices describe nearly a circle in from two 

 to five minutes. Less conspicuous sponta- 

 neous movements take place in Clover, 

 Mimosa, Oxalis ; but they are generally con- 

 cealed by more marked movements due to 

 other causes. Thin walled cells compose 

 the tissue of the active part of the moving 

 organ. " The cells are turgid, and the tissues 

 are in a state of tension, when movements 

 occur it appears that the protoplasm in cer- 

 tain layers of cells permits the escape into 

 the intercellular spaces of a portion of the 

 water of the vacuoles ; it is, however, quickly absorbed 

 again, and the cells rendered thereby turgid, while the 

 escape of water takes place in contiguous layers, to be 

 quickly absorbed again, and so on regularly around the 

 axis of the contracting organ." 



146. Certain other movements depend upon external 

 stimuli. The leaves of many plants assume a 

 position (Fig. 227) at night (sleep) difierent from 

 the ordinary or diurnal position (Fig. 226), in con- 

 sequence of sensitiveness to light. Thus the leaves 

 of Clover, Vicia, Lathyrus, and Honey-Locust fold 

 upwards at night ; those of the Locust and Oxalis 

 downwards. The common petiole of Mimosa turns 

 downwards at night ; that of Phaseolus becomes 

 erect. The leaflets of Mimosa turn laterally for- 

 wards and upwards in the dark ; those of Tephro- 

 sia backwards. The petals of the Tulip, Oxalis, 

 Portulaca, etc., open and close alternately in the 227 



Fig. 226. Diurnal position of the leaflets of the Honey-Locust. Fig. 227. Noc- 

 turnal position of the same. 



