TIME-RELATIONS OF RESPONSE 45 



shown in fig. 17, where the stimulus of sunlight was applied 

 for 2 minutes followed by a period of recovery for 13 minutes. 

 If the stimulus applied on the upper half be strong or 

 long continued, then the excitatory effect is transmitted 

 across the pulvinus to the more excitable lower half. In 

 these circumstances the ' up ' is converted to ' down ' 

 response, on account of the greater contraction of the lower 

 half of the pulvinus. Thus under any form of diffuse 

 stimulation the resultant response in Mimosa is brought 

 about by the differential excitabilities of the upper and 

 the lower halves of the pulvinus. We should also bear 

 in mind that the slight differential contraction-effect in 

 Mimosa leaf is very much magnified by the long petiolar 



Fig. 17. — ' Up ' response (represented by down curve) due to 

 local stimulation of upper half of pulvinus of Mimosa. 



index. There are, again, numerous pulvinar organs whose 

 responsive movements have passed unnoticed. In Desmo- 

 dium gyrans there are two conspicuous pulvini ; the primary 

 pulvinus is at the junction of the petiole with the stem ; 

 there is a secondary pulvinus at the junction of the petiole 

 with the terminal leaflet. The primary pulvinus appears 

 at first sight to be insensitive. But on attaching the primary 

 petiole of Desmodium with the writing-lever, I obtained the 

 series of responses under a very feeble electric shock, as 

 seen in fig. 18. In this particular case the recovery is practi- 

 cally complete in 15 minutes. Other pulvini also exhibit 

 differential contraction under diffuse stimulation. Thus 

 the terminal leaflet of the *bean plant (Vicia Fava) exhibits 



