THE SOLID COMPONENTS OF PLANTS. 239 



in this manner, yet it is constantly renewed ; and, 

 where it remains, the old cuticle cracks as the diam- 

 eter of the stein of the tree, or of the branch, increas- 

 es : it is then gradually pushed outwards, and the ac- 

 cumulation of successive layers, in this manner, forms 

 the rugged coats which characterize many trees, as 

 the Elm and the Oak. Various animals also annually 

 cast their skins, and readily renew parts of it which 

 have been destroyed ; but in vegetables, this occurs 

 on the stems and branches of perennial plants only j 

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

 not renewed after being destroyed. The vegetable 

 epidermis is capable of extention ; but this is less con- 

 siderable than has been supposed ; and as there is a 

 constant renewal, there must be a proportional in- 

 crease or growth of its parts, so that it is not simply 

 extended to enable it to cover a greater portion of 

 surface ; but a new cuticle is added to produce this 

 effect. 



The use of the epidermis is to keep the parts be- 

 neath it together ; and to regulate the perspiration and 

 absorption of the plant. It is calculated also to defend 

 the parts it covers from humidity ; for which purpose, 

 it is covered with a waxy secretion. The powers of 

 the cuticle in regulating these functions is fixed accord- 

 ing to the nature of the plant. In succulent plants, 

 which require much moisture to be retained in then- 

 leaves, the cuticle is so constructed as not to assist ab- 

 sorption, but rather to prevent transpiration. Thus, if 

 a leaf of the Aloe be cut off, it will remain a very long 

 time, even when exposed to the sun's rays, before it 

 shrivels ; but, if in this state it be exposed to damp 

 air, or thrown into water, the absorption is so rapid, 

 that it will regain its original plumpness and size in a 

 few hours. Another use of the epidermis is to pre- 

 vent the destruction of the parts it covers : for, as it is 



