226 ANATOMY AND PHYSIOLOGY OF PLANTS. 



the cellular integument, altered by exposure to the air 

 and light. 



Although we cannot deny the extreme resemblance which the cu- 

 ticle bears to cellular tissue which it covers, it is, nevertheless, 

 a distinct organ : the simple exposure of the cellular membrane will 

 not form epidermis, but on the contrary, it is more apt to be de- 

 stroyed by atmospheric influence, an evident refutation of Mirbtl's 

 opinion. 



25. From the generality of experiments, the cuticle 

 is proved to consist either of a simple and individual 

 layer, as in the epidermis of most plants, or of several 

 distinct and separate or separable layers, as in some. 



In the paper-birch, Du Hamel counted six or more. The same 

 thing is also to be observed in the stem or branches of the currant ; 

 the outer layer of the epidermis of which, after acquiring from age, a 

 tinge of brown, splits into a number of fragments that spontaneously 

 detach themselves to make way for a new layer, which on its first 

 exposure to the air, is of a fine and delicate green. If this layer is 

 now stripped off, it will be found to invest one or two more, which 

 are yet, indeed, colourless and embedded in pulp, being only in a 

 state of preparation for future exposure, when the layer that is now 

 exterior shall have detached itself in its torn. 



26. The precise structure of the cuticle is not yet 

 sufficiently proved ; but whatever may be its real nature, 

 it is very evident, it will admit of the passage of fluids 

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

 nite proportion in every plant ; consequently it must be 

 porous, and the microscope shews what reason would 

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

 ent kinds of plants. 



In very succulrnt plants, as the American aloe, a leaf of which 

 being cut off, will lie for many weeks without drying entirely, and 



