8 The Rev. Patrick Keith o/t 



Til herbs, and in the temporary parts of woody plants, such as 

 the li<avcs and tiowers, the epidermis never det iches itself at all ; 

 which circumstance M. IMirbel adduces as an additional argu- 

 ment in favour of his hypothesis. But to me it seems an argu- 

 ment against it. For, if the air produces such violent effects upon 

 the trunk and branches of woody plants, why does it not produce 

 similar effects upon other plants, or upon other parts of the same 

 plant ? And why is the epidermis of the leaf, flower and fruit in- 

 capable of being again regenerated, if accidentally destroyed ? 

 Till a satisfactory answer can be given to these inquiries, that 

 shall be at the same time compatible with the supposed action of 

 the air in other cases, it is impossible to admit the hypothesis of 

 M. Mirbel. 



But so far is the action of the external air from being the cause 

 and origin of the epidermis, that it is even detrimental to its for- 

 mation. For the re-production of a part that has been acciden- 

 tally destroyed, in cases capable of re-production, is always more 

 easily efiected if the wound be covered closely up*. And hence 

 it is extremely improbable that the epidermis is merely a modifi- 

 cation of the external surface of the parenchyma effected by the 

 influence and action of the air; if rather it is not evidently an 

 organ formed by the agency of the vital principle, even when the 

 plant is yet in embryo, for the very purpose of protectmg it from 

 injury when it shall have been exposed to the action of the air in 

 the process of vegetation. 



Accordingly the developement of the epidermis is found to 

 keep pace with that of the plant which it invests, so that it 

 " grows with its growth and strengthens with its strength," ex- 

 panding in all its dimensions, and accommodating itself with 

 wonderful facility to the augmentation of the inclosed parts, as 

 may be seen in the case of trees and fruits of rapid growth. Its 



* Senebier, Phys. Veg. i. 154. 



expansion 



