EFFECT OF LIGHT UPON PLANTS. Iig9 



or Parenchyma., in which, as I have hinted in the fourth 

 chapter, the chemical operations of the leaves are per>- 

 formed. 



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, inrsomuch 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 vio- 

 let. The common practice of blanching Celery in gar- 

 dens, 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 tliink they err in de- 

 riving it from Ho'xle^ a star. When blanched plants are 

 brought into the light, they soon acquire their natural 

 green colour, and even in the dark they are gieen, if 

 exposed to the action of hydrogen gas. Tulip and 

 Crocus flowers have long ago been observed by Scnne- 

 bier to be coloured even in the dark, apparently be- 

 cause 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 ngrth, and in direct oppo- 



