SPECIAL CASES 251 



remaining for some time in a plagiotropic position. If this acquired 

 dorsiventrality induces a certain tendency to epinastic curvature l , the latter 

 must play some part in the orientation, as must also the primitive positive 

 or negative heliotropism. Short shoots can raise themselves in spite of 

 the action of gravity, whereas long ones unavoidably droop downwards 

 more or less from their attempted position. 



All plagiotropic orientation is not necessarily produced in this way, 

 for many foliage-leaves and other objects are klino-heliotropic and assume 

 their positions mainly in response to the incidence of the light rays. 

 Similarly, under natural conditions negative geotropism and heliotropism 

 may often co-operate in producing a plagiotropic orientation, as for instance 

 when a parallelo-geotropic organ is caused to perform a positively heliotropic 

 curvature by lateral illumination. 



Marchantia 2 . As soon as a permanent dorsiventrality has been 

 induced by the action of light, the subsequent growths take up positions 

 like those assumed by dorsiventral leaves. Thus in strong light the thallus 

 becomes approximately perpendicular to the incident rays, and hence usually 

 assumes a plagiotropic position. This orientation is produced by light 

 independently of the action of gravity, so that illumination from beneath may 

 cause the thallus to become inverted. Hence if the apex is illuminated from 

 the front the thallus tends to bend downwards, but rises up when the light 

 comes from behind the apex 3 . In addition the plagio-phototropic position 

 is assumed when the plants are rotated on a klinostat. Nevertheless the 

 thallus reacts geotropically, and becomes erect in darkness, but more and 

 more horizontal as the illumination increases and the predominant action 

 of light comes into play. It is, however, uncertain whether the thallus is 

 permanently weakly negatively geotropic or whether, as Czapek supposes, 

 illumination affects the geotropic tone as it does that of many runners 

 so that in light the thallus is plagio-geotropic as well as plagio-heliotropic. 



Sachs found that the thallus of Marchantia under ordinary illumina- 

 tion grew at right angles to the direction of a centrifugal force of 34 g., 

 but was inclined at an angle near the centre of the wheel where the 

 force was less. The exact causation of this result remains, however, uncertain 

 until the action of gravity upon a thallus illuminated equally on all sides 

 is known. Czapek 4 supposed that the radial lobes of the thallus developed 

 on the klinostat were diageotropic, but his experiments are not conclusive. 

 Since the upper side of the thallus may become either concave or convex 

 in assuming a plagiotropic position, the latter is evidently not the result 



1 Maige, Ann. d. sci. nat., 1900, 7* se>., T. XI, p. 340; Czapek, I.e., p. 1235; de Vries, Arb. 

 d. hot. Inst. in Wurzburg, 1872, Bd. I, p. 271. 



8 Sachs, Arb. d. bot. Inst. in Wtirzburg, 1879, Bd. II, p. 229 ; Czapek, Jahrb. f. wiss. Bot, 1898, 

 Bd. xxxii, p. 260; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, i, p. 1238. 



s Sachs, 1. c., p. 232. Czapek, 1. c., 1898, p. 263. 



