ORGANS OF GROWTH. 



25 



stinging bristle hairs, seated on a small vesicle or tuber- 

 cule containing poisonous fluid, which is ejected on the 

 hairs being pressed, causing irritation when coming in 

 contact with the skin. 



Armed plants do not appear to possess any special 

 protection ; their fi'uits are in general accessible to 

 animals, especially birds, and their thorny branches are 

 no protection against being browsed on by quadrupeds. 

 Therefore what appeal's to be weapons of defence are not 

 so in nature. 



Baek. (Lat. cortex, Gk. derma.) 



All parts of plants are invested with a skin, called the 

 haj-k or rli'ind, which in permanent stems thickens with 

 age; in leaves, young stems and branches it is thin and 

 soft, being composed of layers of cells (see Structure) of 

 various forms and natures, the surface of the whole 

 covered with a filmy membrane called the cuticle, which 

 with the cells below it forms the outer coat (epidermis), 

 the layer next the wood being called liher. In old trees, 

 as in the elm, it becomes furrowed ; in the oak it is hard 

 and firm, or light as in the cork oak ; in the lime it 

 consists of fibrous layers capable of being twisted into 

 ropes ; and in the paper birch it separates into sheets 

 like pasteboard, from which boats are made. In some 

 species of Daphne, Melalaeuca, and other plants, it con- 

 sists of numerous layers which readily separate from each 

 other, and form sheets like paper ; while that of the lace 

 bark tree of Jamaica is netted, and has been used as a 

 substitute for lace. 



The outer bai'k of some trees separates spontaneously, 

 as in the plane. Palms and their allies have no true 

 bark (see Structure). 



