41] CHAPTER I. GENERAL PHYSIOLOGY. 173 



readily take place, is brought into direct relation with the external 

 air. Transpiration takes place from the cells of the spongy 

 mesophyll into the intercellular spaces, and the watery vapour 

 then escapes from the leaf by the stomata. 



Leaves are adapted not only structurally, but also by their 

 irritabilities, to the performances of their functions. They are 

 sensitive to the directive action of light and of gravity and, in 

 the course of their growth they take up a definite position termed, 

 on account of the predominating influence of light in determining 

 it, the fixed light-position. The response of the dorsiventral leaf 

 to the directive action of gravity, is generally one of diageotropism, 

 that is it places its blade horizontally, with the ventral surface 

 uppermost ; and similarly, its response to light is to expose the 

 upper surface of its blade at right angles to the direction of the 

 incident rays (diaheliotropism). The response of the isobilateral 

 and of the radial leaf to the action of gravity is one of negative 

 geotropism, so that they 

 grow erect ; and to light, 

 one of positive heliotropism, 

 as they tend to direct their 

 apices towards the source of 

 light. 



Changes in the external 

 conditions act as stimuli, 

 which, in many cases, in- 

 duce a movement of the FIG. 125. -Leaf of Oxalis by day (T) and by 



foliage-leaves involving ni ht <> Inthe lter,each leaflet is folded 

 inwards at right angles along its midrib, and is 

 Change of position : most also bent downwards. 



frequently these movements 



are performed by growing leaves, but also sometimes by adult leaves 

 with a permanent motile mechanism. They have been observed in 

 the growing leaves (and cotyledons) of many plants (e.g. Chenopo- 

 dium, Impatiens, Polygonum, Linum, etc.), and in the adult leaves 

 of many Oxalidacese and Leguminosae. 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 surface, 

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



