8 The Rev. Patrick KEITH on 
In herbs, and in the temporary parts of woody plants, such as 
the leaves and flowers, the epidermis never detaches itself at all ; 
which circumstance M. Mirbel 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. abdidit ie Mbit RE 
. But so far is the action of the mere air from being the cause 
ES 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 effected 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 i is yet in embryo, for the very purpose of protecting 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 
-— 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 nelésad parts, as 
may be seen in the case of trees and fruits of rapid growth. Its 
* Senebier, Phys. Veg. i. 154, 
expansion 
+ 
