DIA-HELIOTROPISM AND DIA-GEOTROPISM 



649 



changes which are liable to occur during the lapse of long 

 intervals of time. The action of light is, generally speaking, 

 so predominant over that of the subsidiary factors, that a 

 high magnification of record is not necessary. Under these 

 circumstances, that is to say under low magnification, the 

 record before the application of light is practically a hori- 

 zontal line. Under vertical illumination of the upper side of 

 the leaf, then, deviation above this horizontal represents 

 a positive heliotropic movement, while deflection in the 

 opposite direction signifies the 

 negative. 



But before I proceed to 

 give details of my experi- 

 ments, I shall adduce facts 

 which will show that there is 

 no inherent tendency in the 

 leaves to move in such a 

 manner as to place themselves 

 at right angles to the light. 



Proof of absence of any 

 specific dia-heliotropic ten- 

 dency in leaves. — A Des- 

 modium plant was taken, and 

 the petiole fixed horizontally, 



3 5^ \ 



1 1 1 1 1 



r 5' 10' 15' 20' 25' 



Fig. 260. Movement of Ter- 

 minal Leaflet of Desmodium 

 placing itself Parallel to 

 Incident Horizontal Light 



Fig. 261. Curve showing Time- 

 relations of the Responsive 

 Angular Movement of Terminal 

 Leaflet of Desmodium under 

 Light, as represented in the 

 last figure 



The original angle of 45 between 

 the surface of the leaflet and the 

 direction of incident light was 

 reduced to 13 in the course ot 

 25 minutes. 



the terminal leaflet being at an angle of 45 with the horizon. 

 Sunlight was then made to strike it horizontally (fig. 260). It 

 now the leaflet had a dia-heliotropic tendency, it is clear that 

 its movement would be such as to increase its angle with the 

 horizon to 90 , thus bringing the light to strike it at right 



