ON VEGETABLE PHYSIOLOGY. 59 



of the leaves, there to undergo a new modifica- 

 tion to be hereafter explained. The root therefore 

 may be considered as acting the same part towards 

 the vegetable as the stomach does to the animal ; 

 though the apparatus and the fluid prepared, bear 

 no similarity. 



That part of the plant which ascends from the 

 caudex of the root in a vertical direction into the 

 air, has been denominated the stem or trunk, and 

 its structure is very particularly deserving of your 

 notice. It consists of an external and internal 

 bark both of very singular mechanism, of two 

 species of wood, and of an internal substance 

 denominated the pith. 



The external bark, which has received the dif- 

 ferent appellations of the epidermis, cuticle, false 

 skin, and the like, admits of easy separation, aud 

 is the envelope or outward integument of the 

 plant, extending like the upper skin of animals 

 over its whole surface, so as to completely invest 

 the root, stem, branches, leaves, the flowers (with 

 the exception of the stamen and pistil) and the 

 fruit. In the root and trunk, it is a tough and 

 leathery membrane ; while in the leaves, flowers, 

 and tender shoots, it is a fine colourless and trans- 

 parent film, often as thin as a cobweb. * When 

 examined by the microscope, it seems to be com- 

 posed of a number of slender fibres of membra- 

 nous texture, crossing each other, so as to form 



