Researches on Growth and Moveonent in Plants. 389 



along the stem varies in different cases, from one to several millimetres per 

 second. Transverse conductivity is thus about a hundredth of the longi- 

 tudinal conductivity. 



Uxpt. 12. — I next give an account of experiments on the effect of unilateral 



Fig. 1,3. — Increased turgor due to indirect stimulation, inducing erection oiiMimosa leaf : 

 a, diagram of experiment ; erectile response (shown by down-curve) followed by 

 rapid fall (up-curve) due to transverse conduction of true excitation. 



stimulus of light on the pulvinus of Mimosa. The results will be found to be 

 of much theoretical importance, since this single experiment will give an 

 insight into all possible types of phototropic response. I must first remove the 

 general misapprehension that it is the lower half of the pulvinus that is alone 

 excitable. By careful amputation of the lower half I have been able to show 

 that the upper half is also excitable and contracts under electrical stimula- 

 tion, lifting the leaf. The responsive movement was relatively slower, and 

 the excitability was found to be about one-eightieth that of the lower half. 



In my present experiment a beam of light from a small arc-lamp was 

 thrown on the upper half of the pulvinus. After a latent period of 

 5 seconds, a positive curvature was initiated by the contraction of the upper 

 and expansion of the lower side of the organ. A series of positive responses 

 may thus be obtained under stimuli of short duration. But under continued 

 action of light the excitatory impulse reaches the lower half of the organ, 

 causing a rapid fall of the leaf, thus constituting a mgative response (fig. 14). 

 The thickness of the pulvinus was 1"5 mm. The distance to be traversed 

 to reach the lower half would be about 0"75 mm. ; the time taken by the 

 excitatory impulse to traverse this distance was found to vary in different 

 cases from .50 to 80 seconds. The transverse velocity is thus about Jg- mm. 

 per second. 



