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206 air, in consequence of the abundance of their Cellular Integument, or Parenchyma, in 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 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 étiolation, and our chemists have adopted the term, though I think they err in deriving it from étoile, 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 hydrogen gas. Tulip and Crocus flowers have long