XI] GROWTH CURVATURES 111 



ated with light of too low an intensity to induce the 

 perfection and activity of the chlorophyll, they grow much 

 larger in the dark intervals, suggesting that the feeble 

 light renders something available for growth which could 

 not be utilised in prolonged darkness. The powerful 

 influence of incident light in directing the position of 

 leaves is well seen in window plants (Pelargonium, 

 Tropwolum, &c.) and in wall-climbers (Ivy, Vine, Am- 

 pelopsis, &c.), where the surfaces of the leaves are ex- 

 tended at right-angles to the sun's rays. 



As maturity is approached some leaves exhibit the 

 continued action of growth on their upper surfaces, by 

 becoming convex towards the light ; and others show the 

 effect by becoming as it were pressed on to the surface of 

 the wall up which the plant is growing (e.g. Bignonia). 



Obscure as these phenomena are in their causal re- 

 lations, it is at least clear that differences in the intensity 

 of growth on the two surfaces, and geotropism and helio- 

 tropism, do much to determine the position of the ex- 

 panded leaf; and that local variations in the rate of growth, 

 stimulated by external agents, are the proximal causes of 

 the normal position, which may be regarded as one of 

 equilibrium decided with reference to the resultant of 

 all the forces acting on the plastic organ. Moreover, this 

 position of expansion of the maximum surface to the 

 largest incidence of the solar rays is the most advan- 

 tageous one for the organ. 



So far, however, the assumption and resumption of 

 position have been brought about by growth ; but a time 

 comes when the leaf is fully developed, and, so far as can 

 be perceived, grows no longer, and nevertheless it exhibits 

 movements so-called spontaneous movements which 

 must, at least for the present, be distinguished from those 

 brought about by visible growth. These movements are 



