588 PLANT RESPONSE 



and is probably due to the fact that the excitability of the 

 lower half of the pulvinus is slightly greater than that of the 

 upper. 



We are now in a position to understand the reason of 

 orientation ; for if we suppose light to be incident from a 

 position slightly above the pulvinus, it will curve upwards, 

 owing to positive heliotropism, till it has become parallel 

 with the rays of light. Should there be any over-shooting of 

 this position owing to after-effect, the pulvinus will then 

 begin to curve downwards, because the light will now be 

 acting from below. Permanent equilibrium can thus only be 

 attained when the plant-organ has become parallel to the 

 direction of light. 



The perceptive region in the terminal leaflet of Des- 

 modium. — In connection with the response of the Desmodium 

 leaflet to light, it is interesting to note that the pulvinus is 

 not only the responding, but also the perceptive region ; 

 for, throwing the light on the leaflet alone, and protecting 

 the pulvinus with an opaque shield of black paper, we 

 find that no responsive movement takes place ; conversely, 

 if the pulvinus alone be exposed, and the rest of the leaflet 

 shaded, we observe the normal action. 



Heliotropic response in radial organs. — Having observed 

 the peculiarities of heliotropic movement in a pulvinated 

 organ, I shall now describe the experimental arrangements 

 for studying the same problem in non-pulvinated growing 

 organs. It is understood that there is no essential difference 

 between the two movements. They are both caused by the 

 same contractile effect, due to the stimulus of light, the only 

 difference being that, whereas in the pulvinated organ the 

 recovery is complete, in the growing organ it remains more 

 or less incomplete, the curvature being fixed by growth. 



Magnetically controlled recorder. — The great difficulty 

 which stands in the way of accurate investigation is the 

 question of how to take a continuous time-record of the 

 heliotropic curvature of the growing organ without being 

 under the necessity of using light for purposes of observation, 



