206 EFFECT OF LIGHT UPON PLANTS. 



air, in consequence of the abundance of their 

 Cellular Integument, or ParencJn/ma.m which, 

 as I have hinted in the fourth chapter, the 

 chemical operations of the leaves are performed. 

 That Light has a very powerful effect upon 

 plants has lohg been known, independent oi 

 the remarks of Hales or Ingenhousz. The 

 green colour of the leaves is owing to it, in- 

 somuch 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 blanch- 

 ing 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 natu- 

 ral green colour, and even in the dark they 

 are green, if exposed to the action of hydro- 

 gen gas. Tulip and Crocus flowers have long 



