NEGATIVE HELIOTROPISM 6ll 



seen, however, that such a theory is untenable. The apparent 

 absence in some organs of sensibility to light may perhaps, 

 then, be explicable, not as want of sensitiveness, but as the 

 neutralisation of effect, by the equal excitation of the two 

 opposite sides. 



In the case of certain seedlings of Avena, it has already 

 been shown that the so-called insensitiveness of the lower part 

 of the organ was due to this cause. Many tendrils, again, 

 according to Mohl and others, are heliotropically insensitive. 

 Thus, for example, on subjecting the tendril of Passiflora to 

 lateral sunlight, there is practically no responsive movement ; 

 but as the tendril is a highly conducting organ, we might 

 expect that its responsive movement would be neutralised by 

 the transverse transmission of excitation. It occurred to me 

 that this question, as between a characteristic insensitiveness, 

 and a sensitiveness with equal excitation of two sides, might 

 be tested by artificial reduction of the conducting power by 

 cooling. Under such circumstances, if any sensitiveness 

 existed, a one-sided excitation by light would remain 

 localised, and induce concavity, or positive heliotropic move- 

 ment. On carrying out this experiment, I found that the 

 selected tendril of Passiflora now exhibited a marked positive 

 heliotropic movement by the induced concavity of the side 

 acted upon. 



Negative heliotropism of tendril of Vitis. — The tendril 

 of Vitis is adduced as the type of those organs which exhibit 

 the negative heliotropic effect. 1 therefore undertook an 

 investigation on this organ, to determine whether it would 

 not be possible to explain its negative movement without 

 postulating the existence, in its case, of a specific heliotropic 

 sensibility of negative sign. When sunlight strikes it on one 

 side it is found that it moves away from the light. If, now, 

 this movement be really due to the intensity of stimulus, 

 causing it to be rapidly conducted to the distal side, and at 

 the same time giving rise to the fatigue of the proximal, then 

 we should expect that the application of moderate unilateral 

 illumination would induce the positive heliotropic movement. 



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