5»4 



PLANT RESPONSE 



an exposure of twenty minutes executed five complete 

 oscillations (fig. 237). The notable points in this record are : 

 (1) that a perfectly quiescent organ is made to give multiple 

 response by the stimulus of light ; (2) that molecular 

 sluggishness appears to be gradually removed by the con- 

 tinuous absorption of energy, and the successive responses 

 exhibit an enhanced or ' staircase ' effect ; and (3) that from 

 the tendency of the series of curves to tilt towards the light, 

 the organ is seen to exhibit a resultant positive movement. 



It will thus be seen that we have here not a modifica- 

 tion of an existing movement, but a series of multiple 



responses, with a trend in 

 a particular direction, that 

 is to say towards the 

 light, constituting a posi- 

 tive heliotropic movement. 

 In a growing organ also, 

 which was previously de- 

 void of circumnutation, we 

 shall be able to observe 

 the induction of a simi- 

 lar movement. As in the 

 case of the movements of 

 growth, so also in those of 

 heliotropism, we are often 

 able to detect multiple 

 constituent pulsations, especially at the commencement of 

 response when stimulus is moderate. It will be understood 

 here that, under the unilateral contraction induced in the 

 excited side of the organ by the stimulus of light, a hydrostatic 

 disturbance is set up, the expelled water being forced to the 

 opposite side — a state of things calculated to show pulsation 

 in a marked degree. The final resultant curvature, then, as we 

 have seen under the action of other forms of unilateral stimulus 

 also (p. 521), represents the joint effects of the concavity of 

 the proximal and the convexity of the distal sides. When 

 light acts on the organ continuously for some time, the 



Fig. 237. Multiple Response to Light 

 of Terminal Leaflet of Desmodium 



The moment of application is marked 

 by x . The arrow shows the direction 

 of light, i.e. from above. The 

 numbers in the abscissa represent 

 time in minutes. 



