424 ME. F. DAEWIN ON THE POSITION OF 



the most favourable position for the action of gravity, i. e. 

 horizontal. 



I have dwelt on this view of specific sensitiveness, not only in 

 order to give a clearer notion of Sachs's views, but because it 

 leads on, as he himself states (p. 282), to the belief that light 

 and gravitation act merely as stimuli (" Eeize ") on the organs of 

 plants, the nature of the resulting behaviour of the plant being 

 determined by the inner nature of the organ— as, for example, the 

 uniform stimulus of gravitation is translated by a stem into a 

 command to grow upwards, by a root into a command to grow 

 downwards. And this theory of gravitation and light acting as 

 stimuli is so necessary to the views which I hold on the subject, 

 that I am glad to be able to give it the support of Sachs's opinion. 



The Object of the present Research is, by observation of such 

 organs as place themselves at right angles to the direction of the 

 incident light, to determine whether the '< Transversal-Heliotro- 

 pismus" theory of Frank, or the balancement theories of De 

 Vries and Sachs, is most in accordance with the facts. 



It should be clearly understood that, in speaking of Frank's 

 theory, I do not include his ideas about polarity, by means of 

 which he seeks to explain transverse heliotropism and geotropism. 

 I refer merely to the belief that a special form of heliotropism 

 exists, in accordance with which an organ places itself at right 

 angles to the light instead of parallel to it; and this I take 

 to be the essential and important part of Frank's views. 



Plan of Research.— The method which I have employed is 

 essentially the same as that which has been employed in Sachs's 

 laboratory for the study of ordinary heliotropism. The appa- 

 ratus used for this purpose by Miiller of Thurgau (' Flora,' 1876) 

 consisted of a cylinder rotating slowly about its longitudinal axis, 

 on which the seedlings under observation were grown. The 

 cylinder was placed at a window ; and the plants were protected 

 from all illumination except such as fell on them in a direction 

 parallel to the axis of rotation. The cylinder rotated so slowly* 

 that no centrifugal effect was produced on the plants; but 

 rapidly enough to destroy all geotropic action. The seedlings 

 were therefore entirely free to obey the stimulus of light; and 

 their hehotropic curvature was not liable to be influenced by 

 gravitation, as is the case under ordinary circumstances. The 

 pnnciple on which the stimulus of gravitation can be avoided ha» 

 * One revolution in 20' in my experiments. 



