THE CUTICLE AND EPIDERMIS. 67 



by the distention which the enlargement of the trunk 

 produces ; it is then destroyed entirely, or in part ; and, 

 if we except some fleshy stems which grow slowly, the 

 pellicle which covers the branches of the second or third 

 year presents an appearance different from the cuticle ; 

 it is of a closer tissue, and offers in general a greater 

 thickness. 



This new membrane evidently appears formed, as 

 Malpighi thought, of the external cellules of the cellular 

 tissue, which, being dried by the action of the air, become 

 flaccid, and take a membranous appearance. It is this 

 membrane, sometimes simple, sometimes complex, which 

 bears the name of the Epidermis of Trunks {epiderme des 

 troncs), or the Epidermis, properly so called. Du Petit 

 Thenars, who, in his fifth Essai sur la Vegetation, has 

 well elucidated the formation of the epidermis, remarks, 

 that it would be almost impossible, in every other 

 hypothesis, to conceive the enormous increase which a 

 membrane ought to undergo, which would be supposed 

 the same at the first development of a tree as at an 

 advanced age. The epidermis is simple when the 

 row, or rather the external layer of cellules, is alone 

 dried ; it is double, triple, or multiple, when several rows 

 of cellules are successively dried : this is seen, for ex- 

 ample, in a very high degree, in a Peruvian tree, which 

 UUoa has called Quinales, and of which he says, that 

 having detached more than 150 layers of the epidermis, 

 he had no patience to count more, not having reached 

 the middle of the bark. We can see an analogous cir- 

 cumstance in our White Birch, a branch of which, at its 

 first development, has a cuticle; then it takes a true 

 epidermis; then, as it advances in age, it takes two, 

 three, and even fifteen or eighteen ; and at last finishes 

 by having the bark cracked so as to present interrupted 

 plates of white epidermis on the shreds of its cellular 



F 2 



