TURNING OF LEAVES TO THE LIGHT. 207 



ago been observed by Sennebier to be co- 

 loured even in the dark, apparently because 

 their colour depends on a different principle 

 from the green of leaves. 



Light acts beneficially upon the upper sur- 

 face of leaves, and hurtfully upon the under 

 side ; hence the former is always turned to- 

 Avards the light, in whatever situation the 

 plant may happen to be placed. Trees nailed 

 against a north wall turn their leaves from 

 the wall, though it be towards the north, and 

 in direct opposition to those on a southern 

 wall over against them. Plants in a hot- 

 house all present the fronts of their leaves, 

 and this influences even the posture of the 

 branches, to the side where there is most 

 light, but neither to the quarter where most 

 air is admitted, nor to the flue in search of 

 heat. If the branches of a trained fruit-tree 

 in full leaf be disturbed in their position, the 

 leaves resume their original direction in the 

 course of a day or two. The brighter the 

 day, the more quickly is this accomplished. 

 If the experiment be often repeated, they 

 continue to turn, but more wealdy, and are 

 much injured by the exertion. Black spots 



