EPIDERMIS. 57 



To Wheat, to Rye, and to most kinds of grass, the 

 epidermis is of still greater importance, for at once it 

 supports their stalks, and secures them from injuries, 

 to which, without this defence they would have been 

 continually exposed. The beauty and strength of 

 those straws which are employed in the manufacture 

 of bonnets, and the durability of such as are used in the 

 formation of thatched roofs, may be traced entirely to 

 their exterior covering. In these, and still more abun- 

 dantly in the epidermis of the Rattan and Scouring Rush, 

 Sir Humphry Davy has discovered the existence of 

 flinty earth, and in some cases the quantity is so great 

 as to give fire when struck with steel. It is the flint 

 which enters into its composition that renders the 

 Rush so noxious to animals, so useful in the arts, and 

 it is this which makes the ashes of burnt straw one of 

 the best materials, which can be employed in giving its 

 finest polish to marble. 



The epidermis is sometimes clothed with wool or 

 down, when it becomes an effectual security against the 

 extremes of heat and cold. Thus protected, a few 

 plants linger around the ruins of the volcano, long af- 

 ter they are deserted by their less fortunate compan- 

 ions. The young leaf of the American poplar, Popu- 

 lus granclidentata, affords a fine example of this downy 

 covering, which disappears whenever the enclosed 

 texture becomes so firm as no longer to require it for 

 defence. The wooly covering of the Mullein is more 

 generally known, and its v^ry close analogy to the fur 

 of animals, would warrant the inference that they are 

 designed to answer the same important purposes. 



Thus various are the forms of the epidermis, an 

 organ merely of defence, admirably designed to secure 

 from harm the tender vessels which it encloses, and to 



