NEGATIVE HELIOTROPISM 603 



will be found to offer a satisfactory explanation of various 

 anomalies in heliotropic response. Sinapis, for example, 

 exhibits a strong positive effect in winter, while in hot 

 weather its action is very feeble. It may be supposed that 

 this is due, in some unknown way, to a greater rapidity of 

 growth in warm than in cold seasons. That this, however, 

 cannot be the reason, will be seen from the fact which I have 

 demonstrated, that that contractile response of the plant to 

 external stimulus on which curvature depends is greatest 

 when the rate of growth is at its optimum. The real 

 explanation lies in the fact that the neutralisation, or reversal 

 of normal positive response, caused by transverse conduction, 

 takes place more easily in warmer seasons, the general con- 

 ducting power being then great. This accounts for the 

 feebler positive response in summer, which culminates in 

 certain instances in an actual reversal into negative (p. 623). 



(b) Neutralisation by transverse transmission, with multiple 

 response. — Thus if stimulus be sufficiently strong or long- 

 continued, the positive curvature will become neutralised, 

 and the organ will return to its original position. I have, 

 however, observed an interesting modification of this neu- 

 tralisation, in which it is attended by oscillatory move- 

 ments to and fro about the mean position. We have seen 

 that unilateral stimulus, when its action is long continued, 

 becomes diffused, and thus both sides of the organ become 

 excited. The tissue, moreover, is now possessed of an excess 

 of energy — a condition conducive to the production of 

 multiple response. This fact, together with the periodic 

 and alternate variation of excitability on the two sides, is 

 then found to give rise to oscillatory movements of the kind 

 described. I give below a record which shows the initiation 

 of these oscillatory movements when the organ had been too 

 long subjected to unilateral stimulus. It will be remembered 

 that the pulvinus of the terminal leaflet of Desmodium 

 executes a positive heliotropic movement, the record of 

 which has been given in fig. 238. In winter, when the 

 conductivity of the tissue is feeble, the leaflet curves towards 



