CH. III.] THE STEM AND BRANCHES. \'i7 



its tliickuess until all danger of severe frosts is 

 passed. 



The epidermis regulates the evaporation from a 

 plant, and preserves it in some degree from the de- 

 trimental sudden changes of temperature to which 

 our climate is liable. The birch {Betulus alba), has 

 more films of epidermis than any other European 

 tree ; and it ascends to greater heights in the Alps, 

 and approaches nearer to the frozen zone than other 

 trees of the same climates. 



Immediately below the epidermis occurs the cellu- 

 lar integument (otherwise known as the parenchyma 

 and pulp.) It is a juicy substance ; and, being the 

 seat of colour, is analogous to the rete 7nucosum 

 of man, which is red in the white, and black in 

 the negro : the flesh of fruits is composed of it. 

 Leaves are chiefly formed of a plate of it, inclosed 

 by epidermis. In herbs, succulent plants, leaves 

 and fruits, if it is destroyed, like the epidermis of 

 the same, it remains unrestored ; but in the case of 

 trees and shiaibs, it is regenerated after each re- 

 moval. In leaves it is generally gi'een ; in flowers 

 and fiTiits, of eveiy hue. It is alwa^'s cellular, and 

 evidently acts a part in the secretory system of plants. 



Under the cellular integument occurs the hark, 

 which, in annual plants, or branches of one year s 

 growth, consists of a single layer, scarcely distin- 

 guishable from the wood ; in older stems and branches, 



