Behavior of Stigma Lips. 259 



insect into the mouth of the corolla causes the displacement of 

 the lower lip. This then moves forward, straightening and fin- 

 ally curving upwards to oppose closely the upper lip (Fig. 1). 

 After an interval of about half an hour the lower lip resumes its 

 former position, whether pollen has been received on the stigmatic 

 surface or not. 



Mechanical Displacement the Normal Stimulus. — Merely 

 touching * the papillate stigmatic surface, or bending the papil- 

 lae does not constitute a stimulus. This is shown by the fact 

 that stroking the surfaces with a delicate rod (a broken style) 

 without causing any displacement of the lip as a whole produces 

 no response. The experiment was repeated a number of times, 

 a dozen or more strokes being given in succession. It is hardly 

 possible that the operation could be done without actually dis- 

 placing the papillae in a direction at right angles to their longer 

 axes, from which it would appear that the epidermis of the inner 

 face of the stigma is either not sensitive, or, if sensitive, is in- 

 capable of effecting a visible response to either touch alone or 

 later displacement of the papillae. Further, the stimulus, if 

 effective in any degree, does not pass beyond the epidermis into 

 the tissue below. In this connection it may be noted that the 

 shape of the epidermal cells is such that the mechanical eft'ect of 

 turgor changes will be minimized, namely, by the free walls of 

 the papillae. 



Backward Displacement. — By this is meant displacement 

 of the lip by pressure on its stigmatic surface. The pressure 

 mav be applied so as to move the whole lip or to bend it within 

 a definite zone. The entrance of an insect has the effect of bend- 

 ing the lower lip transversely throughout its whole extent. 



In order to cause response this displacement must be suf- 

 ficiently rapid. I have been able, by steady pressure from the 

 finger-tips, to disj^lace the lower lip (this being more convenient 

 to work wuth than the upper) through a wide angle quite slowly, 

 allowing eqaally slow recovery, without response. The same 

 gentle displacement through a small angle at a temperature of 

 about 70°Fahr. repeated five times at two second intervals, pro- 

 duced, on several trials, no response. If, however, the displace- 



*J. R. Green (Vegetable Physiology, p. 384) says "The stigma of Mimulus — ^will, if either 

 lobe is touched with a fine point, close — ." 



