PULVINUS OF MIMOSA l6l 



shows that the responsive movement was upwards towards 

 the Hght, and that this was initiated after a latent period 

 of 6 seconds. The positive curvature was continuously 

 increased during the whole period of illuminatiouy lasting 

 for 35 seconds or even longer. Hence moderate unilateral 

 photic stimulation of the less excitable upper half of a dorsi^ 

 ventral organ gives rise only to a positive phototropic curvature, 

 The effect of stronger light is, however, strikingly different. 



Experiment 94. Effect of stronger light. — Stronger light 

 from the arc-lamp induced at first an up-movement towards 

 the light after a latent period of 5 seconds. The positive 

 curvature increased continuously for 10 seconds, after which 

 a very striking change occurred. The erectile movement, 

 due to contraction of the upper half of the pulvinus, 

 became reversed into a down-movement, evidently by the 

 arrival of the excitatory impulse across the boundary line 

 between the upper and lower halves of the organ. The 

 slow fall of the leaf at the beginning was due to the suc- 

 cessive contraction of the cortical tissue of the lower half 

 of the organ ; a short while after this the contractile fall 

 of the leaf became very abrupt, the rapidity of fall becoming 

 so great that a scratch, and not a dot, was produced as 

 the writer was jerked off the plate (right record, fig. 89). 



Moderate stimulation of the upper half of the pulvinus 

 is thus seen to give rise to a positive curvature towards the 

 Hght, while stronger stimulation gives rise to a positive 

 followed by a negative. What can be the reason for this 

 behaviour ? The probable explanation is, that under 

 moderate stimulation the downward percolation of exci- 

 tation along the vertical layers of cortical cells is a slow 

 process on account of the resistance offered by the semi- 

 conducting tissue. But under stronger stimulation the 

 partial block becomes forced, and the excitation not only 

 percolates through the upper half of the pulvinus but also 

 reaches the lower half. The moment of arrival of excitation 

 at the upper boundary of the lower half of the organ is, as 

 already stated, signahsed by the inversion of the curve. 



