CH. Xl] EXPANSION OF LEAF 109 



to be viewed always in proper perspective with regard to 

 the chief one carbon-assimilation or photo-synthesis. 



Nor is this generalisation affected by the facts that 

 particular leaves may be modified to perform quite differ- 

 ent duties from those touched upon e.g. they may act 

 as climbing organs, floats, insect- traps, &c. and that 

 organs other than leaves may be employed for carbon- 

 assimilation, either in addition to or in place of the 

 typical leaves. 



Consequently we are here concerned with a complex 

 piece of physiological machinery i.e. with an organ 

 the normal working of which can only be understood by 

 studying its normal structure and relations to the natural 

 environment on the one hand, and its behaviour in ex- 

 periments on the other. 



When the young leaf begins to emerge from the bud, 

 it expands rapidly and is at first concave on the side next 

 the centre of the bud, with its petiole or midrib nearly 

 erect. As extension of the lamina proceeds, the upper 

 surface grows somewhat more quickly than the lower, and 

 this process results in the gradual throwing downwards 

 and outwards of the flat organ, until its plane is more or 

 less horizontal, and extended in some direction nearly at 

 right-angles to that of gravitation and to the incident 

 rays of the sun, or at least in a plane which exposes the 

 surface to the maximum incidence of the sunlight. 



Since compound as well as simple leaves effect this 

 extension of their surfaces in darkness, as well as in light, 

 and since the same position is regained by torsions of 

 the petioles, or petiolules, if the leaves are displaced, the 

 directive stimuli are probably to be found both in the 

 interior of the plant and in its environment. 



Experiments show that some of these curvatures are 

 in part geotropic i.e. the directive stimulation has 



