_Experiment 126._--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. But under continued action of light, the excitatory impulse reached the lower half of the organ, causing a rapid fall of the leaf, and a _negative_ curvature. The arrival of transmitted excitation at the more excitable distal half of the organ is clearly demonstrated by the very rapid down-movement, seen as the up-curve in the record (Fig. 120). In sensitive specimens this movement is so abrupt and rapid, that the writing lever is jerked off above the recording plate before making a dot on it. The thickness of the pulvinus was 1·5 mm., the distance which the excitatory impulse has to traverse to reach the lower half would thus be about 0·75 mm. The period for transverse transmission of excitation under strong light was found to vary in different cases from 50 to 80 seconds. The velocity of transmission of excitation in a transverse direction through the pulvinus is about 0·011 mm. per second, which is not very different from 0·010 mm. per second in the stem (p. 282). [Illustration: FIG. 120.--Record of effect of continuous application of light on upper half of pulvinus of _Mimosa_ leaf. Note erectile response (positive curvature) followed by neutralisation and pronounced reversal into negative due to transverse conduction of excitation. Up-movement shown by down curve, and _vice versâ_.] Returning to the main experiment we find that: (1) As a result of unilateral action of light, there was positive phototropic curvature which lasted for 50 seconds. (2) Owing to the internal conduction of excitation the positive effect underwent neutralisation by the excitatory contraction of the distal side. This neutralisation depends on four factors: (_a_) on the intensity of the stimulus, (_b_) on the conductivity of the organ in a transverse direction, (_c_) on the thickness of the intervening tissue, and (_d_) on the relative excitability of the distal as compared to the proximal side. The extent of positive curvature also depends on the pliability of the organ. (3) In anisotropic organs where the distal side is physiologically the more excitable than the proximal, the internally diffused excitation brings about a greater contraction of the distal, and the _positive_ phototropic curvature becomes reversed to a very pronounced _negative_. The effect of the internally diffused stimulus is thus the same as that of external diffuse stimulus.