SPECIAL CASES 253 



position only slight epinastic curvature takes place, whereas vertical shoots 

 curve until they assume a horizontal position both in light and in darkness. 

 This certainly points to the existence of a diageotropic irritability which 

 is not modified by illumination ; but in any case the diageotropism is bound 

 up with the induced dorsiventrality, since the radial shoots appear to be 

 ortho-geotropic. It is possible that unilateral illumination may be capable 

 unaided of producing a diaphototropic orientation, although Sachs 1 con- 

 sidered the plagiotropism to be due to negative heliotropism and geotropism, 

 whereas Czapek 2 supposes it to result from phototropism, diageotropism, 

 and photonasty. 



THE PLAGIOTROPIC BRANCHES OF HERBS AND TREES. These appear 

 in the case of Cucurbita Pepo 3 , Linaria cymbalaria 4 , and Tropaeolum majus 5 

 to resemble the ivy, in that unilateral illumination induces dorsiventrality. 

 The latter is, however, so feeble in Tropaeolum that we may equally well 

 suppose the plagiotropic position to result from the opposed action of 

 diaheliotropism and negative geotropism. Permanently dorsiventral plagio- 

 tropic organs are, like radial and temporarily dorsiventral ones, unequally 

 responsive to light and gravity. Naturally the action of light becomes of 

 predominant importance in the case of photosynthetic organs or surfaces, 

 as well as in the stem when the position of the latter is mainly responsible 

 for that of the leaves. This applies to the plagiotropic shoots of Atropa 

 Belladonna^Pilea^Pellionia, Goldfussia anisophylla^ and Selaginella*, although 

 in part the influence of gravity which is exercised even upon foliage-leaves 

 may predominate. Considerable uncertainty exists, however, in many 

 cases. Thus it is not known whether the dorsiventrality and plagiotropism 

 of Polygonatum multiflorum 7 is due to light, to gravity, or to both. 



Obliquely ascending radial or dorsiventral branches which bear leaves 8 

 capable of self-orientation usually show only feeble phototropic reactions, 

 lateral illumination producing little or no heliotropic curvature. The 

 geotropic irritability may, on the other hand, be mainly responsible for 

 the direction of growth assumed, this being always at a definite angle to the 

 perpendicular. The primary and secondary branches of many herbaceous 

 and woody plants may, however, show but feeble geotropic reactions, and 

 have little or no power of plagiotropic orientation. In such cases the 

 branches spread in all directions, and continue any direction of growth 



Sachs, Arb. d. bot. Tnst. in Wiirzburg, 1879, Bd. II, p. 266. s L. c., p. 258. 



Czapek, Flora, 1898, p. 427; Noll, Landw. Jahrb., 1901, Ergzbd., p. 425. 

 Oltmanns, Flora, 1897, p. 26. 

 Sachs, I.e., p. 271. 



Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 265. Cf. also Wiesner, Ber. d. bot. Ges., 

 1902, p. 321 ; Sitzungsb. d. Wien. Akad., 1902, Abth. i, Bd. CXI, p. 733. 



7 Cf. Vochting, Bewegungen d. Bliithen u. Fruchte, 1882, p. 148; Frank, Die natiirl. wagerechte 

 Richtung von Pflanzentheilen, 1870, p. 21. 



8 Cf. de Vries, Arb. d. bot. Inst. in Wurzburg, 1872, Bd. I, p. 271 ; Frank, 1. c. 



