426 ME. V. DARWIN ON THE POSITION OF 



power of placing themselves at right angles to incident light 

 is growing normally in the open air and lighted from above, 

 its leaves will be horizontal. Let the plant be now made to 

 rotate on a klinostat so that the axis of rotation coincides 

 with the axis of the plant. Also let the direction of the 

 incident rays of light be parallel to the axis of rotation, so 

 that the morphologically upper side of the leaves is illuminated 

 by rays striking them at right angles, just as they were when 

 the plant grew on the ground. Then, if the normal horizontal 

 position is the result of a balance between geotropism (positive 

 or negative) and any other force — epinasty, hyponasty, positive 

 or negative heliotropism — it is clear that, geotropism being de- 

 stroyed by the rotation, the balance cannot be maintained. 



If, on the other hand, the horizontal position of leaves is due 

 to transverse heliotropism (or diaheliotropism *), there is no 

 reason why the leaves should not remain at right angles to the 

 incident light. The stimulus which determines the position of 

 the leaves is still the same, since the light is still at right angles 

 to the surface of the leaf. Experiments planned after this type 

 have been carried out on several plants with various results, as 

 will be shown in detail. 



Definition of Terms. 



Light parallel to axis. — In describing the fundamental experi- 

 ment I spoke of the direction of the light as parallel to the axis of 

 the klinostat. In the actual experiments the klinostat stood close 

 to a window, so that at any given moment the light struck the 

 plants on the klinostat obliquely ; but as the plant was in con- 

 tinuous rotation about an axis perpendicular to the panes of glass 

 in the window, it is obvious that the light would be practically 

 parallel to the axis. 



Let x and y, fig. 3, be the cotyledons of a seedling plant (with 

 hypocotyl k) growing in a box of earth, B, which is fixed to the 

 end of the spindle of the klinostat Jc k. Let the arrow 1 in 

 fig. 3 represent the direction of the light striking the leaf * ; 

 now let the klinostat rotate through half a turn, so that x is 

 downwards, fig. 4, and now the arrow 2 will represent the direc- 

 tion of the light ; so that x will be equally exposed to light in the 

 direction of the two arrows 1 and 2 in fig. 4, the resultant being 



* This term has been proposed, in ' The Power of Movement in Plants,' as 

 more convenient. 



