IRRITABILITY. 517 



is not, however, always constant throughout a Natural Order, 

 nor throughout a genus, nor even in individuals of the same 

 species as Darwin found in Loasa aurantiaca. The direction 

 of twining may even be reversed in successive internodes 

 of the same stem : this occurs, according to Darwin, occasion- 

 ally in Loasa aurantiaca, and habitually in Scyphanthus 

 elegans. 



Finally, in order to complete the subject, we have to 

 enquire into the influence of light upon climbing stems. 

 In the first place, with regard to the tonic influence of light, 

 Sachs has ascertained that Phaseolus multiflorus and Ipomoea 

 piirpurea can twine in continuous darkness, whereas Dioscorea 

 Batatas (Duchartre, de Vries) and Mandevillea suaveolens 

 (Duchartre) cannot twine under these circumstances. With 

 regard to the directive influence of light, von Mohl con- 

 cluded that twining stems are positively heliotropic, but 

 less markedly so than other stems. Wiesner detected slight 

 positive heliotropism in the stems of species of Convolvulus, 

 Ipomcea, and Calystegia, but not in PJiaseolus multiflorus 

 nor in Cuscuta. Darwin observed the same in Ipomoea 

 jucunda and in Lonicera brachypoda : the circumnutating 

 apex of the former described the semicircle towards the 

 light in one hour, and the semicircle away from the light 

 in four hours and a half; the apex of the latter described 

 the corresponding semicircles in 2 hrs. 37 min. and 5 hrs. 

 23 min. respectively. Baranetzky's observations agree in the 

 main with the foregoing, but he found that the internodes 

 were positively heliotropic only when young ; when older 

 they became negatively heliotropic. 



BIBLIOGRAPHY. 



Contact. 



Darwin ; Climbing Plants, 1875. 



de Vries ; Arb. d. bot. Inst. in Wiirzburg, I, 1873, P- 3 2 - 

 Sachs; Text-book, 2nd English edition, 1882, p. 870. 

 Dutrochet ; Comptes rendus, xix, 1844, and Ann. d. sci. nat., se"r. 3, 

 II, 1844. 



