MOVEMENTS OF LEAVES 97 



He then goes on to describe his observations on Abrus ; the 

 structure of the leaf, more especially the course of the vascular 

 bundles, is first dealt with, and then an explanation of the action 

 of light is given. Needless to say, in view of the state of physical 

 science at this period, his explanation, although ingenious, is 

 wide of the mark. He wrote that " Light is subtile, active, and 

 penetrating : by the smallness of its constituent parts, it is 

 capable of entering bodies ; and by the violence of its motion, 

 of producing great effects and changes in them. These are not 

 permanent, because those rays which occasion them, are, in that 

 very action, extinguished and lost. 



" Bodies may act on light without contact ; for the rays may 

 become reflected when they come extreamly near : but light can 

 act on bodies only by contact ; and in that contact the rays are 

 lost. The change produced in the position of the leaves of plants 

 by light, is the result of a motion occasioned by its rays among 

 their fibres : to excite this motion, the light must touch those 

 fibres ; and where light touches, it adheres, and becomes im- 

 mediately extinguished The raising of the lobes in these leaves 



will be owing to the power of those rays which at any one 

 instance fall upon them: these become extinguished; but others 

 immediately succeed to them, so long as the air in which the 

 plants stands, is enlightened." 



Although it was not until 1822, when Dutrochet pointed out 

 the true significance of the pulvini, Hill recognized that these 

 structures were concerned with the movements of the leaflets, 

 not only in the case of Abrus^ but also in Mimosa. He remarked 

 that " It is on the operation of light upon these interwoven 

 clusters of fibres [which are placed at the bases of the main rib, 

 and of the several foot-stalks of the lobes], that the motion of the 

 leaves in gaining their different positions depends ; and conse- 

 quently, the motion itself is various according to the construction 

 of these fibres. 



" In the Abrus they are large, and of a lax composition ; 

 consequently the lobes are capable of a drooping, an horizontal, 

 and an oblique upward position : in the Tamarind, and the 

 broad-leaved Robinia, they are more compact, and hence all the 

 motion of which those leaves are capable, is an expanding open 



o. B. 7 



