400 



COMPARATIVE ELECTRO-PHYSIOLOGY 



shown in fig. 236, where the first is seen in the up-curve, 

 which thus indicates the joint action of contraction in the 

 upper and expansion in the lower, giving rise to an erectile 

 movement. After an interval of one minute the excitatory 

 effect is seen to have reached the lower half, giving rise now 

 to a reversed or down movement, which, on account of the 

 greater excitability of the lower, is seen to carry the leaf 

 downwards, much below its original position. The dotted 

 portion of the curve shows the after effect on the cessation of 



the stimulating light. It is 

 here interesting to observe 

 that it is possible to obtain 

 an after-effect which is posi- 

 tive and of opposite sign to 

 the true excitatory effect, this 

 positivity being due to the 

 increased internal energy 

 consequent on the absorption 

 of stimulus. 



In fig. 237 are shown the 

 electrical effects consequent 

 on stimulation by light in 

 another specimen of the 

 pulvinus of Mimosa. The 

 electrical contacts are made 

 this case, one with the 



FIG. 236. Mechanical . Response of 

 Pulvinus of Mimosa to Continuous 

 Action of Light from Above applied 

 at Moment marked 4- 



Positive heliotropic movement caused 

 by excitation of upper half neu- 

 tralised by transmission to distal 

 side, and ultimately reversed owing 

 to greater excitability of lower half. 

 Dotted line represents recovery on 

 cessation of light. Note final erec- 

 tion of leaf above original position 

 as after-effect of absorbed stimulus. 



Ill 



lower half of the pulvinus and 

 the other with a distant in- 

 Stimulus of light is applied, as in the 

 This experimental method 



different point. 



last case, on the upper half. 



is free from the objection which has been urged against 



Dr. Waller's experiments on green leaves, that the result 



was complicated by the direct action of light on the electrode 



itself. It should be pointed out, however, that the presence 



of such photo-electric action is more important theoretically 



than practically, being of relatively small amount. A further 



complication which arises from the direct action of light on 



