STRUCTURE OF MOTILE ORGANS. 



629 



We have now only to do with the true movements of sleeping and waking, which 

 are thus induced by paratonic light-stimuli and their after-effects; and in the first 

 place we shall consider those leaves which we may look upon as the most completely 

 organised in this direction, namely, the compound leaves of the Leguminosae, 

 Oxalidece, and others of that type. It will then be relatively easy to make intelligible 

 the phenomena in question in the case of other leaves where special motile organs 

 are not developed, and in the case of flowers. 



It is in the first place important to obtain a clear idea of the motile organs them- 

 selves, and this is probably to be obtained in no way better than by examining the 

 common Garden Bean {Phaseolus ??iultißorus and P. vulgaris), a single leaf of which 

 is represented in Fig. 366 in the nocturnal position. At a is the motile organ of 

 the leaf-stalk proper, by means of which it is connected with the stem ; at b and c 



FIG. 366.— Leaf of llie Scarlet Runner {Phaseolns mullißoriis) in the nocturnal position ; a the large motile organ at the base 

 of the leaf-stalk d d ; *<- the small motile organs of the three leaflets eee. 



are the motile organs of the individual parts of the leaf, the so-called leaflets. The 

 portions d d oi the leaf-stalk are stiff, and immovable on their own account, as are 

 also the laminae of the leaflets e e. It is clear that if the motile organ a, which is now 

 in the nocturnal position, elongates a Httle on its upper side, it must necessarily curve 

 downwards, and the stalk d d naturally falls ; if at the same time the motile organs 

 b and c undergo a slight elongation on their lower sides, and accordingly an upward 

 curvature, the individual leaflets e e become raised, and we will assume this to occur 

 so that they all come to lie in one and the same plane. In this case, then, the leaf 

 attains the diurnal position, and it may be incidentally mentioned that the change here 

 described is induced by the increasing light in the morning ; the opposite change, 

 which carries the leaf back to the sleeping position again, results from the decrease 

 in the brightness of the light in the evening. Although to superficial observation, 

 it is the movement of the petiole and of the lamina which is most striking, this is 



