PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 633 



only as long as it takes the stimulus to reach the distal side 

 of the pulvinus. When strong concentrated sunlight is 

 applied from above, therefore, the duration of this upward 

 movement is reduced, and it appears only as a slight twitch. 

 The same effect is produced under less intense light when, 

 as in thinner pulvinated organs, the distance to be traversed 

 by stimulus is not so great. 



(&) The diurnal sleep of Oxalis. — In consequence of strong 

 vertical illumination, as at noonday, negative heliotropic 

 response, with downward 

 folding of the leaves, takes 

 place in those pulvinated 

 organs in which there is 

 internal diffusion of sti- 

 mulus, and in which the 

 lower side of the pulvinus 

 is much more excitable 

 than the upper. As ex- 

 amples of this, we shall 

 take the cases of Oxalis 

 and Biophytum. In the 

 former, if light of moderate 

 intensity, say from a lamp, 

 be applied on the dorsal 

 or upper surface, an up-/ 

 ward, or positive, heliotro- 

 pic curvature is induced. 

 If the same light be 

 applied below, a positive 

 heliotropic movement, in this case downwards, is again 

 induced ; but the fact that the lower side is much the 

 more excitable is seen when the upper and lower surfaces 

 are excited simultaneously, for there is now a downward 

 movement (fig. 257, a). Thus external diffuse stimulation 

 induces downward movement, and we shall also find that 

 stimulus internally diffused has the same effect. This is seen 

 by throwing a strong beam of sunlight on the upper surface, 



P'ig. 257. Responses of Oxalis to Sun- 

 light 



(a) Shows greater excitation of lower half 

 and downward response when both 

 upper and lower are simultaneously 

 subjected to stimulus of light ; {&) shows 

 downward or negative response owing 

 to internal diffusion of stimulus when 

 upper surface only is acted on by 

 strong light. Arrows indicate the di- 

 rections of incident light. 



• 



