134 CHAP. XIII. DIA-PHOTOTROPISM 



This was demonstrated by the experiment with the stem of 

 Mimosa, in which Hght was appUed on the proximal side, oppo- 

 site to the indicating motile leaf on the distal side. After 

 the preliminary erectile response, due to indirect stimulation, 

 the leaf exhibited a fall on account of the conduction of 

 the excitatory impulse across the stem {cf. Experiment 70) 

 which induced a diminution of turgor at the distal side. 

 This would antagonise the effect induced at the proximal 

 side by direct stimulation, and neutralise it. 



The extent of the neutralisation will therefore depend 

 (i) on the transverse conductivity of the tissue, and (2) on 

 the intensity and duration of the incident stimulation. 

 Other things being equal, neutralisation will be incomplete 

 in a thick, and complete in a thin, stem. The following 

 experiments were undertaken in verification. 



Experiment y^. Partial neutralisation. — The moderately 

 thick stem of Dragea volubilis was exposed to the unilateral 

 action of an arc-light. The first effect was a positive 

 phototropic curvature, which, after reaching a maximum, 

 was partially neutralised under continued action of the 

 light for 2 hours. 



Experiment 79. Complete neutralisation. — The thin stem 

 of a young seedling of Phaseolus does not offer so great 

 a resistance to the transverse conduction of excitation as a 

 thick stem. When light from Pointolite of 30 candle-power 

 was unilaterally applied on the seedling, the maximum 

 positive curvature was induced in about 2 minutes. This 

 curvature was, however, completely neutralised under the 

 continued action of light for 5 minutes. 



Strong and prolonged unilateral stimulation does not, 

 however, end in mere neutralisation, which places the organ 

 at right angles to light, regarded as the dia-phototropic 

 position. The transformation is carried still further ; thus, 

 three stages of phototropic action may be distinguished : 

 positive at the beginning, neutraHsation or dia-phototropic 

 attitude -as the intermediate, and negative phototropism as 

 the final effect. How^ is this final transformation effected ? 



