98 EPIDERMIS. [LECT. HI. 



plates^ or in layers, the old cuticle is reduced into 

 powder* 



Although the epidermis is not cast off from 

 all plants in this manner, yet, it is constantly 

 renewed ; and, where it remains, the old cuticle 

 cracks as the diameter of the stem of the tree, or 

 of the branch, increases: it is then gradually 

 pushed outwards, and the accumulation of suc- 

 cessive layers, in this manner, forms the rugged 

 coats which characterize many trees^ as the Elm 

 and the Oak. This renewal of the epidermis in 

 vegetables is similar to what takes place in ani- 

 mals. The snake, for example, casts his skin an- 

 nually, as do also the crab, the lobster, the spi- 

 der and many other insects ; and the cuticle of 

 the human body often peels off, particularly after 

 some diseases, as scarlet fever for instance, and 

 on the application of acrid matters to the skin. 

 In animals, however, when destroyed, it is again 

 regenerated ; but in vegetables, this occurs on the 

 stems and branches of perennial plants only ; for 

 on annual plants, and on the leaf and flower, it 

 is not renewed after being destroyed. The ve- 

 getable epidermis is capable of extension ; but 

 this is less considerable than has been supposed ; 

 and as there is a constant renewal, there must be 

 a proportional increase or growth of its parts, so 

 that it is not simply extended to enable it to cover 



