PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 63 



the lower half of the organ, the responsive movement of the 

 pulvinus being due to that factor alone. In the course of the 

 present work, however, I have frequently stated that the 

 upper half also was excitable, and that the usual responsive 

 movement was due to the differential excitability of the two. 

 It is now easy, using localised stimulus of light, to submit 

 this question to a crucial test. 



First I took a record of the responsive movement of the 

 leaf of Mimosa, when the upper half of the pulvinus alone 

 was subjected to stimulus of sunlight. Fig. 255, a, shows 

 the moderate positive, 

 or in this case upward, 

 movement, which was 

 the result of this sti- 

 mulation. By means 

 of a properly inclined 

 mirror light was now 

 thrown vertically up- 

 wards, so as to strike 

 the lower half of the 

 pulvinus. The con- 

 sequent positive re- 

 sponsive movement, 

 in this case down- 

 wards, is seen to be 

 much stronger (fig. 

 255, b\ on account of 



the greater excitability of the lower half of the organ. The 

 differential character of the responsive movement under 

 externally diffused stimulation is shown in fig. 255, r, which 

 is a record of the response given by the pulvinus when both 

 its upper and lower sides were simultaneously acted upon by 

 light. 



It will be seen that in this record, then, we have a case of 

 response to externally diffused stimulus. We shall next 

 observe the effect of stimulus which has become internally 

 diffused, owing to conduction from the upper to the lower 



Fig. 255. 



Responses of Mimosa to Sunlight of 

 not too long Duration 



Light acting on pulvinus from ahove ; (/>) light 

 acting on pulvinus from below ; (c) light acting 

 simultaneously from above and below. Dotted 

 line represents recovery on cessation of light. 



