4fifi SENSITIVENESS TO LIGHT. CHAP. IX. 



detected in the accuracy of their bending, whether 

 they stood with their broad or narrow sides facing 

 the light, or in any intermediate position ; and so it 

 was with the cotyledons of Avena sativa, which are 

 likewise oval in section. Now, a little reflection will 

 show that in whatever position the cotyledons may 

 stand, there will be a line of greatest illumination, 

 exactly fronting the light, and on each side of this 

 line an equal amount of light will be received ; but 

 if the oval stands obliquely with respect to the light, 

 this will be diffused over a wider surface on one side 

 of the central line than on the other. We may there- 

 fore infer that the same amount of light, whether 

 diffused over a wider surface or concentrated on a 

 smaller surface, produces exactly the same effect ; for 

 the cotyledons in the long narrow box stood in all 

 sorts of positions with reference to the light, yet all 

 pointed truly towards it. 



That the bending of the cotyledons to the light 

 depends on the illumination of one whole side or on 

 the obscuration of the whole opposite side, and not on 

 a narrow longitudinal zone in the line of the light 

 being affected, was shown by the effects of painting 

 longitudinally with Indian ink one side of five coty- 

 ledons of Phalaris. These were then placed on a table 

 near to a south-west window, and the painted half was 

 directed either to the right or left. The result was that 

 instead of bending in a direct line towards the window, 

 they were deflected from the window and towards the 

 unpainted side, by the following angles, 35, 83, 31, 

 43, and 39. It should be remarked that it was hardly 

 possible to paint one-half accurately, or to place all 

 the seedlings which are oval in section in quite the 

 same position relatively to the light; and this will 

 account for the differences in the angles. Five coty- 



