164 CHAP. XV. PHOTONASTIC PHENOMENA 



half of the pulvinus. The same characteristic is observed 

 when strong hght acts from above. The arrival of the 

 excitatory impulse at the uppermost layer of the lower 

 half induces a reversal of response from up to down 

 movement. This excitatory fall is, however, slow at the 

 beginning, becoming very rapid as soon as the excitation 

 reaches the highly excitable layers lower down [cf. fig. 89). 

 The two results confirm the conclusion that the position of 

 the most excitable tissue is somewhere in the middle of the 

 lower half of the pulvinus. The results of experiments on 

 which this conclusion is based are by no means accidental, 

 but were obtained under all modes of stimulation applied 

 on the upper and lower halves of the organ. 



To facilitate explanation, I give a diagrammatic repre- 

 sentation of the longitudinal section of the pulvinus, in 

 which the number of layers has been reduced. The vas- 

 cular bundle F divides the upper from the lower half of 

 the organ. The upper cortex extends from a to F ; b to d 

 represents the extent of the lower cortex ; c represents the 

 hypothetical intermediate layers, the moto-excitability of 

 which is exceptionally high (fig. 91). 



Speaking generally, one half of the pulvinus, commonly 

 the lower half, is more excitable than the other ; it is this 

 more excitable half that has a determining influence on 

 the movement of response. The positive impulse due to 

 indirect stinmlation causes an enhancement of turgor and 

 expansion of this half, giving rise to a positive erectile 

 movement. The excitatory impulse reaching it later causes 

 contraction and fall of the leaf. 



Now, if there is a gradation of excitability in the lower 

 half, then the most excitable as well as the most contractile 

 layer c will function as the essential responding layer. 

 The existence of a specially excitable layer in the lower 

 half of the pulvinus is indicated by the foregoing experi- 

 ments ; for percolation of excitation, either from above or 

 below, to such a layer would account for the abrupt change 

 from a slow to an abrupt fall. 



