158 



Sir J. C. Bose and Mr. S. C. Guha. 



that in plants sensitive to light the operative factors in the diurnal move- 

 ment are : — 



a. The variation of geotropic action with changing temperature. A rise 

 of temperature is found to inhibit the geotropic action ; a fall of temperature 

 accentuates it. In consequence of this the leaf, subject to geotropic action, 

 undergoes a periodic up-and-down movement ; the maximum fall of the leaf 

 takes place at thermal noon, which is about 2 p.m., the maximum rise is at 

 thermal dawn, about 6 A.M. 



b. The action of light is, generally speaking, antagonistic to that of 

 temperature. In the forenoon, rise of temperature causes a fall of the leaf, 

 but continuous light acting from above tends to raise it. The rapid 

 diminution of light towards evening acts virtually like a stimulus, causing 

 an abrupt fall of the leaf. 



The diurnal movements of Mimosa and Helianthus exhibit four phases 

 which are very similar : — 



(1) The leaf, owing to fall of temperature, erects itself from 2 to 5.30 p.m., 

 or thereabouts. 



(2) After 6 p.m. there is a rapid diminution of light, and the leaf undergoes 

 a sudden fall, which continues till about 9 p.m. 



(3) After 9 p.m. the leaf begins to erect itself with the fall of temperature, 

 the maximum erection being attained at thermal dawn, which is at 6 A.M., 

 approximately. 



(4) In the forenoon the leaf is acted on by two antagonistic reactions, 

 the effects of rising temperature and of increasing light, the effect of rise 

 of temperature being predominant. The leaf thus continues to fall till 

 thermal noon, which is about 2 p.m. 



5. Torsional Response to Lateral Stimulus. 



I shall now refer to a very important type of responsive movement 

 induced by lateral stimulus. A stimulus is called lateral, when it acts 

 either on the right or the left flank of a dorsiventral organ. I shall 

 presently show that a dorsiventral organ responds to a lateral stimulus 

 by torsion. That this effect is universal will be demonstrated by experi- 

 ments on the " sensitive " Mimosa, and the " ordinary " plant Helianthus. In 

 order to eliminate the effect of the weight of the leaf, and also for obtaining 

 record of pure torsion, the petiole is enclosed in a hooked support of glass, 

 with a smooth internal surface (fig. 2). Friction and the effect of weight 

 are thus practically eliminated ; the circular support prevents any up or down 

 movement, yet allows freedom for torsional response. I have recently 



