RESPONSIVE GROWTH-CURVATURES IN PLANTS 735 



to say, a negative response. It was thus made clear that the 

 three types of response — positive, negative, and dia-hcHo- 

 tropic — are not due to three different specific sensibiHties. 



It has been pointed out, further, that these considerations 

 explain why it happens in many cases that, while moderate 

 stimulation induces a considerable responsive movement, 

 stronger stimulation, instead of increasing this, actually 

 neutralises it. It is due, as we have seen, to the transverse 

 conduction of stimulus by the tissue, that the positive effect is 

 counteracted or reversed. This explanation has been shown to 

 account satisfactorily for various cases apparently anomalous. 



Certain tendrils are regarded as heliotropically insensi- 

 tive. For example, the tendril of Passifiora when acted on 

 by sunlight shows little or no responsive movement. On 

 artificially diminishing the transverse conduction, however, 

 by the application of cold, I have shown that it exhibits 

 the ordinary positive responsive movement. The tendril 

 of Vitis, again, which is supposed to be endowed with 

 a specific sensibility of negative character, has also been 

 shown to exhibit the normal positive response under light of 

 moderate intensity. The modifications of transverse con- 

 ductivity which are brought about by age and season, with 

 their consequent appropriate variations of response, are seen 

 in Tropceolum. A very young tissue, as a general rule, owing 

 to the fact that the fibro-vascular elements are not fully 

 developed, is a bad conductor of stimulus, which therefore 

 remains localised at the point of application. Hence young 

 plants exhibit movements of positive response, whereas older 

 plants, owing to transverse conduction, with its effect of 

 neutralisation, appear to be little affected b}' light. In con- 

 nection with this it must also be borne in mind that the 

 power of contraction declines with age. The characteristic 

 effect of season, again, results from the fact that the con- 

 ducting power of a tissue is at its feeblest in autumn and 

 winter, and correspondingly greater in spring and summer. 

 In autumn, therefore, stimulus remains localised, and 7^/7^- 

 pceoluui and Ivy during that season respond to heliotropic 



