TURNING OF LEAVES TO THE LIGHT. 157 



That Light lias a very powerful effect upon plants 

 has long been known, independent of the remarks of 

 Hales or In2[enhousz. 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^ 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 ctoik, 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 acrainst a north wall turn their leaves from the 



o 



wall, though it be towards the north, and in direct 

 opposition to those on a southern wall over against 



