100 EPIDERMIS. [LECT. nr. 



demonstrate, that the greatest activity, irritability, 

 and degree of vital energy reside, if that part be 

 wounded to any considerable extent, so that the 

 external air gets access to it, exfoliation, and the 

 death of the part, and sometimes that of the whole 

 plant, follow ; the cuticle forming, as Sir J. E. 

 Smith elegantly expresses it, " a fine, but essential 

 66 barrier between life and destruction*." 



Mirbel-f- combats the idea of the epidermis 

 being a distinct organ, and supposes it to be the 

 external layer of the cellular membrane con- 

 densed, and altered by exposure to the air and 

 light. But although I admit that the cuticle be 

 nearly the same as the parietes of the cellular tissue 

 which it covers, yet, it is nevertheless a distinct 

 organ. The simple exposure of the cellular mem- 

 brane will not form epidermis ; but, on the contrary, 

 when the cellular substance is exposed, it is more 

 apt to exfoliate ; and when the wound becomes 

 healthy, it is then only that cuticle is reproduced. 

 During this process the new epidermis proceeds 

 from the sides of the wound, gradually extending 

 over it, in the same manner as in a wound of the 

 human body. The very close connexion of the 

 epidermis and the cellular substance can be no 

 argument against our opinion ; for, although the 



* Introduction to physiological and systematical Botany, 

 2dedit. p. 18. 



f Eltmem de Pkys.^veget. l ere Partie, p. 35. 



