252 TROPIC MOVEMENTS 



of dorsiventral epinasty or hyponasty. It is not yet, however, certain 

 whether a thallus grown in darkness may not perform a photo-epinastic 

 curvature when exposed to light, the curvature increasing as the light 

 becomes more intense. 



As was first observed by Mirbel \ illumination of the under side causes 

 this to become concave until the upper surface is exposed to the light, the 

 curvature being at first towards the light and then away from it. Sachs 2 

 considered the plagiotropism of Marchantia to be due to the interaction 

 of negative geotropism with a positive heliotropism of the lower side, and 

 epinasty in the upper one, whereas Czapek 3 supposed it to result from the 

 co-operation of diaphototropism, photo-epinasty, and a diageotropism 

 varying according to the illumination. The stalks of the fructifications of 

 Marchantia are parallo-geotropic and parallelo-heliotropic, and owing to their 

 high heliotropic irritability Sachs (1. c.) found that they assume a position 

 nearly parallel to the incident rays when obliquely illuminated. 



THE PROTHALLUS OF FERNS is also oriented mainly by its plagio- 

 heliotropism, and reacts in the same way as does Marchantia when 

 illuminated from beneath 4 . Since the induced dorsiventrality is labile, 

 however, the new growths soon have their dorsiventrality reversed, and the 

 orienting movement ceases or may never be shown if it is delayed too long. 



HEDERA HELIX 5 . Unilateral illumination induces labile dorsiventrality 

 in the stems of this plant, and so produces the plagiotropic position of the 

 shoot. Hence the ascending stems press themselves against a vertical wall 

 and curve over the top of it away from the light until the free ends bend 

 downwards by their own weight. The hypocotyl as well as the inflorescence 

 axes are, however, radial and ortho-geotropic 6 . 



When illuminated equally on all sides by rotation on a klinostat the 

 shoots remain radial, while the dorsiventrality may be reversed by illuminat- 

 ing the under-surface. Owing to the slowness of curvature and the 

 relatively rapid reversal of the dorsiventrality, an ivy-shoot when illuminated 

 from beneath curves only slightly towards the light and then curves away 

 from it 7 . It is not known, however, whether geotropic stimuli play any part 

 in the orientation. The shoots of Hedera do actually react geotropically, 

 but according to Sachs 8 they are negatively geotropic, whereas according to 

 Czapek 9 they are diageotropic. Sachs states, however, that in a horizontal 



1 Mirbel, Rech. anat. et physiol. sur le Marchantia, 1835 (reprint from Nouvell. Ann. du 

 Museum d'Histoire nat., T. l). Cf. also Czapek, 1898, 1. c., p. 262. 



3 L.c., p. 239. * L. c., 1898. 



* Leitgeb, Flora, 1877, p. 174; 1879, p. 317. 



5 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, p. 257 ; Czapek, Jahrb. f. wiss. Bot., 1898, 

 Bd. xxxii, p. 258; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, i, p. 1236; Oltmanns, Flora, 1897, 

 p. 26. 



6 Czapek, 1. c., 1895, p. 1236. 7 Sachs, 1. c., p. 267. 



8 Id., p. 269. Czapek, 1. c., 1898, p. 358. 



