IRRITABILITY. 469 



the root be impaired by the removal of a ring of cortical 

 tissue, the root will not curve when placed horizontally. Both 

 Brunchorst and Wiesner have investigated the effect of centri- 

 fugal force on decapitated roots. They both found that when 

 the roots are in a moist chamber, they curved outwards under 

 the influence of this force like normal roots, but Brunchorst 

 observed that they did not so curve when the chamber was 

 loosely filled with damp sawdust. Brunchorst concludes from 

 this that the curvature observed in the moist chamber is not 

 a response to the stimulus of the centrifugal force, but is a 

 purely mechanical result which is prevented by the intro- 

 duction of the moist sawdust. On the whole, then, the 

 evidence is clearly in favour of Darwin's view that geotropic 

 irritability is localised in the tips of roots. 



We go on now to consider the geotropic phenomena of 

 bilateral organs, and we begin with those of isobilateral 

 organs. The long narrow flattened leaves of Monocotyledons 

 such as Iris are already familiar to us as examples of organs 

 of this kind. All that we have to say with regard to them is 

 that they are orthotropic and negatively geotropic. 



We have to consider, finally, the geotropic phenomena of 

 those bilateral organs which are dorsiventral. We find, in 

 the first place, that many organs which, when exposed to 

 light, take up the plagiotropic position characteristic of dorsi- 

 ventral organs, do not do so in the absence of light, but that, 

 under these circumstances in which gravity is the one ex- 

 ternal directive influence which acts upon them, they grow 

 erect, they are orthotropic. This is the case, as Frank has 

 shewn, with the creeping shoots of Lysimachia Nummularia, 

 Polygonum aviculare, Airiplex latifolia and other plants, with 

 radical leaves, and with the thalloid shoots of Marchantia. 

 Other instances are doubtless afforded by the shoots of Tro- 

 pseolum and of the Ivy to which reference was made in our 

 discussion of heliotropism (p. 443). We know already that, 

 in the case of the creeping shoots above-mentioned, their 

 dorsiventrality is induced by the action of light, and we 

 cannot be surprised to learn that in the absence of light 

 they should exhibit the negative geotropism characteristic of 



