REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 711 



(d) Torsional response.— Another interesting type of 

 response occurs when an anisotropic or dorsi-ventral organ 

 is stimulated laterally. Under such conditions, a responsive 

 torsion is induced, by which the less excitable side is made 

 to face the external stimulus. The extent of the response 

 depends on the intensity of stimulus, and the differential 

 excitability of the anisotropic organ. In dorsi-ventral organs, 

 the plane of anisotropy is fixed. But in certain climbing 

 plants, this plane revolves in a positive or negative direction, 

 and under the internal activity of growth, an autonomous 

 torsional movement is thus observed. The opposition of the 

 effects of internal energy and external stimulus is here seen, 

 when such an organ is uni-laterally acted on, say by light. 

 The autonomous torsional movement is then found to be 

 retarded, or even reversed. 



(e) Death response. — There is another curious phenomenon 

 of response, which takes place at a certain definite point. 

 When an organ is gradually raised in temperature, the 

 internal energy is increased, and the organ exhibits a re- 

 sponsive expansion, if radial by elongation, or if pulvinated 

 by erection ; but when the death-point is reached, a sudden 

 and irreversible molecular change takes place, attended by 

 an excitatory contraction. In the curve of thermo-mechanical 

 response we here find a sharply defined point of reversal, 

 which affords us an exact index of the death-point. This 

 death-point is very definite in plant-organs under normal 

 conditions ; in phanerogamous plants it is very near 6o° C. 

 Physiological modification of the tissue, moreover, may be 

 gauged by the transposition of the otherwise definite death- 

 point (p. 185). 



(/) Thermographs of regional death. — Just as there is a 

 definite point of reversal in the thermo-mechanical curve, so 

 there is also a point of discoloration which is, under standard 

 conditions, at a determinate interval from the death-point. 

 A particular region, physiologically changed, may thus be 

 thermally 'developed/ and made to exhibit as a thermo- 

 graph, a picture of localised physiological variation (p. 184). 



