428 CHAP. XXXVII. GENERAL REVIEW 



(2) the stage of increasing positive curvature reaching a 

 maximum ; (3) the stage of neutrahsation ; and (4) the 

 stage of reversal to negative. The first part of the curve is 

 negative, due to physiological expansion induced by sub- 

 minimal stimulation. The curve then crosses the abscissa 

 upwards ; in the second stage, the susceptibility for excita- 

 tion, feeble at the beginning, increases very rapidly with 

 increasing intensity or duration of stimulation. The 

 positive phototropic curvature then attains its maximum. 

 At the third stage neutralisation occurs. And, finally, at 

 the fourth stage the curve crosses the zero-line downwards, 

 the phototropic curvature being reversed to negative. 



Phototropic torsion. — Lateral stimulation of any kind 

 induces a torsional response in a dorsiventral organ, the 

 direction of torsion being such that the less excitable half 

 of the organ is made to face the stimulus. Conversely, the 

 direction of an incident stimulus can be determined from 

 the observed direction of torsional movement. The torsional 

 movements of leaves and leaflets of many plants are ex- 

 plicable on the above general principle. The excitatory 

 efficiency of two different stimulations can be compared by 

 the Torsional Balance, by observing the resulting effect 

 when the two stimuli act simultaneously on opposite flanks 

 in the plane of junction of the more and less excitable 

 halves of the anisotropic organ (p. 257). 



Photonastic curvature. — There is no line of demarcation 

 between phototropic and photonastic movements ; con- 

 tinuity exists between them. By the application of a 

 method of recording the effect of percolation of excitation 

 through the pulvinus of Mimosa, a gradation of excitability 

 has been discovered in the different tissues of the lower half 

 of the organ (p. 163). In an organ with pronounced physio- 

 logical anisotropy, in which the distal side is far more 

 excitable than the proximal, the transverse conduction of 

 excitation brings about a greater contraction of the distal 

 side. The sequence of response is then positive, neutral, 

 and very pronounced negative. Application of stimulus 



