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 Rh 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 immediately 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 consequently, 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