634 



PLANT RESPONSE 



when the leaflet is found to respond by depression (fig. 255, b ). 

 Here, then, we have a repetition of those movements which we 

 have already seen in the case of plagiotropic stems, where 

 strong illumination of whichever surface always had the same 

 effect, that is to say the induced concavity in the lower. 



if) Diurnal sleep of Biophytum. — In the case of Oxalis 

 and in that of Mimosa the responsive movement is more 

 or less continuous ; but in such a pronouncedly multiply- 

 responding organ as that of Biophytum, the effect of internally 

 diffused stimulus on the more excitable lower half of the 

 pulvinus is to induce depression by a series of multiple 



responses. This is well seen in 

 the following record (fig. 258), 

 where under the action of strong 

 sunlight from above the leaflet is 

 undergoing a progressive fall. A 

 record of a similar effect in the 

 case of Biophytum is seen in fig. 

 123, the fall being in that case 

 represented by an up curve. The 

 specimen there was somewhat 

 sluggish, and there were three 

 pulsations in the course of ten 

 minutes, making an average of 3*3 

 minutes to each. In the present 

 instance, however, there were six 

 pulsations in the course of fifteen 

 minutes — each, that is to say, having an average period of 2*5 

 minutes. Similarly in Averrhoa the fall of the leaflet occurs 

 in a pulsating manner, an instance of which is given by 

 Darwin. 1 



The directive versus non-directive action of light. — It 

 will be well at this point to enter into the question of arbitrary 

 distinction which is commonly made as between heliotropic 

 action in radial organs and the same action in the anisotropic 

 or dorsi-ventral. It is clear, however, that such a distinction 



Fig. 258. Negative Multiple 

 Response in Biophytum 

 when Acted on from Above 

 by Strong Sunlight 



Darwin, Movements of Plants, p. 333. 



