IRRITABILITY. 443 



ties ; that the dorsal is negatively and the ventral positively 

 heliotropic. This explanation was mentioned by Sachs, 

 in his paper on orthotropic and plagiotropic plant-organs, 

 but it appeared to him that the assumption was an impos- 

 sible one. Wiesner, however, is more bold, and accepts and 

 attempts to justify it. We shall not follow him in this, but 

 will endeavour to shew that the phenomena can be satis- 

 factorily accounted for without making this very doubtful 

 assumption. 



Such an explanation is not far to seek, and it is to Sachs 

 that the first suggestion of it is due. In the paper mentioned 

 above, when speaking of the influence of light upon the 

 Marchantia-thallus, Sachs says : " So far as I can at present 

 apprehend the facts, this negative heliotropism of the Mar- 

 chantia-shoots, and that of many other shoots which behave 

 in the same way, is the same phenomenon as the epinasty of 

 foliage-leaves described by de Vries." Taking this statement 

 in connection with Detmer's results (see supra, p. 383) that 

 light promotes the epinasty of foliage-leaves, we arrive at once 

 at an explanation of the phenomena in question. The dorsi- 

 ventral organs which we have been considering are photo- 

 epinastic ; that is, that when exposed to intense light their 

 dorsal surfaces grow more rapidly than their lower. 



We will here digress for a moment and revert to the appa- 

 rent reversal of the heliotropic properties of orthotropic organs 

 to which allusion was made a short time ago (p. 429). It 

 was then pointed out that the effect of intense unilateral 

 illumination was to induce dorsiventrality in previously 

 radial shoots (Ivy, Tropaeolum). This being the case, we 

 see that their apparent negative heliotropism is nothing 

 more than the photo-epinasty which we have found to obtain 

 in the dorsiventral organs now under consideration. It is 

 probable that this explanation applies also to the apparent 

 negative heliotropism of the tendrils of Vitis and Ampelopsis 

 to which allusion was made above (p. 429). 



We have come to the conclusion, then, that dorsiventral 

 organs are not negatively heliotropic, and we may now go on 

 to enquire if they are positively heliotropic ; that is, if under 



