108 PHYSIOLOGICAL VIEWS. 



Ejyidermis^ or cuticle, is tlie skin r)f the membrane wliicli 

 extends over the surface of plants. There is a striking anal- 

 ogy between animal and vegetable cuticle, or skin. In the 

 animal it \'aries in thickness, from the delicate film which 

 covers the eye, to the thick skin of the hand or foot, the 

 coarser co\'ering of the ox, or the hard shell of the tortoise. 

 In the vegetable it is exquisitely delicate, as in the covering of 

 a rose-leaf; or luird and coarse, as in the rugged coats of the 

 elm and oak. The cuticle serves for protection from external 

 injuries, and regulates the proportion of absorption and perspi- 

 ration through its pores. It is transparent as well as porous, so 

 as to admit to the cellular integument the free access of light 

 and air, while it excludes every substance which would be in- 

 jurious. 



■ a. It is to the cuticle of wlieat, oat, rye, and some of tlie grasses, that we are 

 indebted for straw and Leghorn hats. In their manufacture the cellular texture 

 is scraped aAvay, so that nothing remains but the cuticle. It has been ascertained 

 that the outer bark of many of the grasses contains silex, or flint ; — in the scouring 

 rush {Eqnisctum), the quantity of silex is such, that housekeepers find it an excel- 

 lent substitute for sand, in scouring wood or metals. A peculiar property of the 

 cuticle is, that it is not subject to the same changes as the other parts of bodies : 

 it is, of all substances found upon animal or vegetable matter, the most indestructi- 

 ble. The cuticle is sometimes, like the skin of animals, clothed with wool, as in the 

 leaf of the mullein ; the pericarp of the peach has a downy cuticle. 



126. Cellidar integument is situated beneath the epidermis 

 or outer skin of the bark ; it is filled with a resinous substance, 

 usually green in young plants. It envelops the branches, as 

 well as trunks of trees, and herbaceous stems ; it extends into 

 roots, but there it neither retains its green color, nor decom- 

 poses carbonic acid gas. It is the seat of color, and in this 

 respect analogous to the cutis^ or true skin of animals, which is 

 the substance situated under the cuticle, and is black in the 

 Negro, red in the Indian, and pale in the American. This 

 herbaceous envelope of the trunks of trees after a time dries, 

 appearing on the surface in the form of a cuticle, and often 

 cleaves olf ; it is renewed internally from the cambium. 



127. Cortex. — Immediately under the cellular integument, 

 we find the true bark, which, in plants of only one year 

 old, consists of one simple layer ; but in trunks of older trees, 

 it consists of as many layers as the tree has numbered years. 

 The cortex is formed of bundles of fibers called cortical vessels. 



The peculiar virtues or qualities of plants chiefly reside in 

 the bark. Here we find the resin of the fir, the astringent 

 principle of the oak, and the aromatic oil of the cinnamon. 

 The i7iner layer of the hark is called the liber ; from liher., a 

 book, on account of its fine and thin layers resembling the 



Epidorrais — a. Uses of the epidermis. — 126. Cellular integument. — 127. Cortex — Liber. 



