NEGATIVE HELIOTROPISM 



609 



intermediate neutral, in consequence of the increasing internal 

 diffusion of stimulus with increasing intensity of stimulating 

 light. This I shall do by a continuous record taken from a 

 single plant under changing conditions of increasing illumina- 

 tion, in which record, further, we shall be able to follow all 

 the phases of responsive change, from positive to negative. 

 Taking a hypocotyl of Sinapis tugra, I subjected it to the 

 unilateral action of light from a sixteen-candle-power incan- 

 descent electric lamp, placed at a distance of 10 cm. from 

 the specimen. The plant, hitherto quiescent, began to move 

 towards the light, as shown 

 by the up curve in the 

 record (fig. 247), the maxi- 

 mum being attained in 

 the course of fifty minutes. 

 The intensity of incident 

 stimulus was now in- 

 creased, by bringing the 

 lamp to a distance of 6 cm. 

 from the specimen, at the 

 moment marked with the 

 downward arrow. It will 

 be seen that this resulted 

 in a process of neutralisa- 

 tion of the preceding re- 

 sponse, and that this 

 became complete in the 

 course of a further exposure of seventy minutes, the hypo- 

 cotyl being then erect and free from curvature, having thus 

 placed itself at right angles to the incident light. Still 

 stronger illumination of sunlight was now applied, at the 

 point marked x . This induced, as is seen in the down curve, 

 a very marked reversed or negative heliotropic response. 



Thus, in other words, in an identical organ, under different 

 conditions of illumination, the plant turning towards the 

 light exhibits the positive licliotiopic, at right angles to the 

 light the dia-licliotropic, and away from the light the negative 



i< R 



Fig. 247. 



Response of Hypocotyl of 

 Sinapis nigra 



The first curve shows positive response 

 induced by incandescent, ^electric lamp 

 at distance of lo cm. Increased in- 

 tensity of light applied at arrow (|) 

 hy bringing lamp to distance of 6 cm. 

 causes neutralisation. Reversal, ornega- 

 tive response, when sunlight applied 

 at X . 



