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PART III. PHYSIOLOGY. 



[41 



leaves at the same time approaching each other, with the result 

 that they are protected from injury by cold in consequence of 

 excessive radiation of heat : with a rising temperature and an in- 

 creasing intensity of light, the leaves assume the "day-position," 

 presenting their upper surfaces to the zenith. But the day-position 

 is frequently liable to modification, with a view to the reduction 

 of transpiration and to the protection of the chlorophyll from the 

 action of too intense light, by movements which diminish the leaf- 

 area exposed to the direct rays of the sun ; and so, in some cases, 

 the edge, and not the upper surface, is presented to the sun : these 

 movements are designated "diurnal sleep" or parahcliotropism. 



Some foliage-leaves, but only such as have a special motile 

 mechanism, respond by movement to the stimulus of a touch. 



FIG. 126 (after Duchartre). Leaves of Mimosa pudtca: A normal diurnal position ; 

 B position assumed on stimulation. 



This is the case in the " sensitive plants," such as Mimosa pudica 

 and some other species : the leaflets of the pinnate leaves of these 

 plants close together when touched, or when the plant is shaken, 

 and they are thus protected to some extent from injury by hail, 

 rain, or even wind. Other instances of movement in response to 

 touch are afforded by the "carnivorous" genera, Dionsea and 

 Aldrovanda, in which, when an insect alights on the upper sur- 

 face of the expanded leaf and touches the sensitive hairs, the two 

 lateral halves of the blade suddenly close together, like a hinge, 

 with the midrib as the axis. 



Sensitiveness to long-continued contact is manifested by the 



