604 



PLANT RESPONSE 



the light to the maximum extent possible, and remains in 

 that position as long as the light acts. But we have seen 

 that in summer the stimulus is more likely to be internally 

 transmitted to the distal side, the positive effect being thus 

 gradually neutralised. Thus, in the course of an experiment 

 during the summer on the pulvinus of the terminal leaflet of 

 Desmodium, I found, on subjecting it to sunlight from above, 

 that for the first forty minutes the leaflet rose continuously, 

 its tip having moved during that time through a little more 

 than 4 cm. After this there was induced, instead of the 

 continuous movement upwards, a pulsatory movement up 



and down (fig. 246). After a series 

 of such movements the leaflet was 

 gradually depressed, the former 

 positive curvature being thus 

 neutralised. 



The supposed localisation of 

 sensitiveness to light, and the 

 transmission of excitatory effect. 

 I have fully explained the man- 

 ner in which the effect of stimulus 

 of light applied at a given point is 

 transmitted to the distant growing 

 organ, and the mechanics by which 

 the curvature is induced. In con- 

 nection with this, a peculiar phe- 

 nomenon has been observed, which has led to the belief 

 that in seedlings, like that of Avena sativa, the zone for 

 the perception of heliotropic stimulus is confined to the 

 upper region, or tip of the shoot. This conclusion is based 

 on Darwin's observations on the unilateral effect of light 

 on these seedlings. It was found that, generally speaking, 

 when the lower part of the cotyledon was alone exposed to 

 the unilateral light — the upper part being covered with a cap 

 of tinfoil or with an opaque glass tube — there was little 

 curvature induced ; but when such light was allowed to act 

 on the upper part of the seedlings the curvature was con- 



Eig. 246. Positive Heliotropic 

 Movement of Terminal Leaf- 

 let of Desmodium Converted 

 by Strong and too Long- 

 continued Stimulus of Light 

 into Oscillatory Movement 



