TURNING OF LEAVES TO THE LIGHT. 157 



That Light has a very powerful effect upon plants 

 has long been known, independent of the remarks of 

 Hales or Ingenhousz. The green colour of the leaves 

 is owing to it, insomuch that plants raised in darkness 

 are of a sickly white. It has even been observed that 

 when light is admitted to the leaves through different 

 glasses, each tinged of a different prismatic colour, the 

 plant is paler in proportion as the glass approaches 

 nearer to violet. The common practice of blanching 

 Celery in gardens, by covering it up from the light, is 

 an experiment under the eyes of every one. This 

 blanching of plants is called by the French Etiolation, 

 and our chemists have adopted the term, though I 

 think they err in deriving it from etoile, a star. When 

 blanched plants are brought into the light, they soon 

 acquire their natural green colour, and even in the 

 dark they are green, if exposed to the action of hy- 

 drogen gas. Tulip and Crocus flowers have long-ago 

 been observed by Sennebier to be coloured even in the 

 dark, apparently because their colour depends on a 

 different principle from the green of leaves. 



Light acts beneficially upon the upper surface of 

 leaves, and hurtfully upon the under side; hence the 

 former is always turned towards 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 



