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 argument in favour of his hypothesis. But to me it seems an argument 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 incapable 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 formation. For the re-production of a part that has been accidentally destroyed, in cases capable of re-production, is always more easily efiected if the wound be covered closely up[1]. And hence it is extremely improbable that the epidermis is merely a modification 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 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 keep pace with that of the plant which it invests, so that it "grows with its growth and strengthens with its strength," expanding 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.
