Behavior of Stigma Lips. 261 



(a) A single forward thrust against the middle of both 

 upper and lower lip produced no response, nor was there when 

 the stimulus was repeated after 30 seconds. The lip closed 

 after a single backward thrust. 



(b) Stead\- pressure for 20 seconds repeated after 20 

 seconds was followed by no response. If the pressure was con- 

 tinued steadily for one minute, there was displayed a tendency 

 of the lip to remain in the displaced position, though repeated 

 trials produced inconstant results. In all cases, a single back- 

 ward thrust caused closing. 



(c) Five thrusts repeated at intervals of one second, no 

 response. Ten thrusts with similar intervals caused a displace- 

 ment from the original position, but no active closing move- 

 ment. Normal stimulus caused the usual response 



The forward thrust causes a compression of the irritable 

 cells, while the backward thrust, which alone is effective in caus- 

 ing response, stretches the tissues. 



The displacement resulting from the former is, I believe, 

 purely passive. It seems then, that the stimulus which cause? 

 closing is a sufficiently vigorous stretching of the motile tissues 

 consequent a backward thrust. Brown and Sharp * found that 

 the closing response in Dionaea follows the compression of cer- 

 tain cells at the bases of the sensitive hairs by the bending of 

 these, but that release from the bent position does not constitute 

 a stimulus. In both Dionaea and Diplacus, therefore, a mere 

 shock appears to be insufficient since the factor of direction en- 

 ters in. In order to harmonize my own conclusion, that the sen- 

 sitive cells must be compressed, we may say that the plasmatic 

 membrane must be stretched. It is probable that, if a turgid cell 

 is distorted by pressure applied unequally (as the basal cells of a 

 trichome such as that in Dionaea would be byflexure) a portion 

 of the plasmatic membrane would be stretched, unless free to glide 

 on the inner surface of the cell wall, a condition which probably 

 does not obtain. Indeed, Haberlandt t showed that the sensitive 

 cells of the convex side of the zone of bending in the Dionaea hair 

 lengthened 21 percent of the original dimensions. On the other 

 hand the bending back of the stigma-lip oiDiplacus may have the 



*Brown. W. H.. and Sharp, \.. W. The closing Response in Dionaea. Bot. Gaz. 49: 290-302 

 April, 1910. 

 tPhysiologische Pflanzenanatomie, 4th Ed. p. 481. 



