Page:An introduction to physiological and systematical botany (1st edition).djvu/48

 18 exposed to a burning sun, with very rare supplies of rain.

This part allows also of the passage of air, as is proved by experiments on the functions of leaves. Light probably acts through it, as the cuticle is a colourless membrane. We know the effects of light to be very important in the vegetable œconomy.

But though this fine membrane admits extraneous substances, so as to have their due effect upon the vegetable constitution, according to fixed laws, it no less powerfully excludes all that would be injurious to the plant, either in kind or proportion. Against heat or cold it proves, in general, but a feeble defence; but when clothed with hair or wool, it becomes a very powerful one. Against the undue action of the atmosphere it is so important a guard, that, when any tender growing part is deprived of it, the greatest mischiefs ensue. It forms in the Vegetable, as well as the Animal, a fine but essential barrier between life and destruction.

Some have imagined that the cuticle gave form to the vegetable body, because it sometimes being over tight causes contractions on