14 OF THE CUTICLE OR EPIDERMIS. 



The cuticle admits of the passage of fluids from 

 within as well as from without, but in a due and defi- 

 nite proportion in every plant: consequently it must be 

 porous ; and the microscope shows, what reason would 

 teach us to expect, that its pores are different in dif- 

 ferent kinds of plants. In very succulent plants, as 

 Aloes, a leaf of which being cut off will lie for many 

 Aveeks in the sun without drying entirely, and yet when 

 partly dry will become plump again in a few hours if 

 plunged into water, the cuticle must be very curiously 

 constructed, so as to admit of ready absorption, and 

 very tardy perspiration. Such plants are accordingly 

 designed to inhabit hot sandy countries, where they are 

 long 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 co- 

 lourless membrane. We know the effects of lii^ht to 

 be very important in the vegetable oeconomy. 



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 atmo- 

 sphere it is so important a guard, that, when any tender 



