CHAPTER I. GENERAL PHYSIOLOGY. 



689 



podium, Impafciens, Polvgonum, Limim, etc.), and in the adult 

 leaves of many Oxalidaceae and Leguminosse. The common 

 feature of these movements is that they serve to vary the area of 

 surface presented to the sky by the leaf. They are commonly 

 known as " sleep-movements," or nyctitropic movements, because 

 they are usually associated with the alternation of day and night. 

 With a falling temperature and a diminishing intensity of light 

 the leaves assume the "night-position," presenting a diminished sur- 

 face, generally only the edge, to the zenith, the leaflets of compound 

 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 increasing 

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

 their upper surfaces to the 

 zenith. But the day-posi- 

 tion is frequently liable to 

 modification, with a view to 

 the reduction of transpira- 

 tion and to the protection of 

 the chlorophyll from the ac- 

 tion 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 paraheliotropism. 



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

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

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

 other species, Biophytum (Oxalis) seusitivum, JEscliynomene indica, 

 Neptunia oleracea : 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 surface 

 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. 



Fro. 471. Leaf of Oxalis by day (T) and by 

 night (N). In the latter, each leaflet is folded 

 inwards at right angles along its midrib, and is 

 also bent downwards. 



