586 



PLANT RESPONSE 



Desmoctium which exhibited such remarkable sensitiveness 

 had been specially chosen, as being in the least favourable 

 tonic condition. And yet I could not even strike a match 

 near this plant without inducing responsive movements. 

 This will indicate the extreme sensitiveness of certain plants 

 to light, and show that it is necessary to make our 

 observations of induced movements without its aid. The 

 manner in which this was done will be described presently. 



Merging of multiple in continuous response. — We 

 found that light acting from above on the terminal leaflet of 

 Desmoditim gyrans gave rise to multiple 

 responses. As the light was acting con- 

 stantly on the upper half, there was a 

 cumulative contractile effect on that half. 

 The consequence of this was a trend of 

 the series of curves towards the light, or 

 an incipient positive heliotropic move- 

 ment. I shall now proceed to show how 

 these constituent multiple movements 

 may often, if not always, merge into one 

 continuous movement. 



For this experiment I used the same 

 leaflet as in the last, the only difference 

 being that I now applied the strong 

 stimulus of sunlight from above. The 

 response induced is seen in fig. 238. 

 During the first impact of the stimulus a 

 pulsatory movement may sometimes be 

 observed. But the response soon becomes 

 a continuous movement upwards. Generally speaking, the 

 constituent multiple marks are to be seen under feeble 

 or moderate stimulation, and a continuous movement when 

 the stimulus is strong. In the present case, the average 

 rate of movement of the tip of the leaf was almost 

 1*5 mm. per minute (fig. 238). On the stoppage of light 

 there was persistence for some time of the after-effect of 

 light. This was succeeded by recovery. The persistence 



Fig. 238. Response 

 of Terminal Leaflet 

 of Desmodium to 

 Strong Light from 

 Above 



Abscissa gives time in 

 minutes, and or- 

 dinate movement in 

 millimetres. 



