BARK. 147 



ment proves the importance of this fluid to the existence of the 

 plant. 



The Proper Juices of Vegetables. This division compre- 

 hends all the fluids furnished by the plant except the sap and 

 cambium ; as oils, gums, &c. These are the product of the 

 cambium, as in the animal, tears are secreted from blood. 

 The secretions, carried on by the vegetable glands from the 

 cambium, are of two kinds; 1st, such as are destined to remain 

 in the plant, as milk, resins, gums, essential and fixed oils ; 

 2d, such as are destined to be conveyed out of the plant, as 

 useless or incongruous ; these consist chiefly of vapours and 

 gases exhaled from flowers, and are sometimes called excre- 

 tions. 



LECTURE XVIIL 



Bark, Wood and Pith. 



WE have exhibited to your view the minute .discoveries made 

 by the help of the microscope in the solid parts of vegetable 

 substances ; and we have also noticed those important fluids, 

 the circulation of which appears to constitute the life, and pro- 

 duce the growth of plants. We have now to consider the solid 

 parts already described, as composing the body of the .vegeta- 

 ble, and collected under three forms of Bark, Wood, and Pith. 



Bark. The bark consists of the epidermis, cellular integu- 

 ment and cortex. 



1st. Epidermis is the skin or membrane which extends over 

 the surface of every vegetable. It is also called the cuticle, a 

 name which anatomists have given to the external covering of 

 the animal body. There is a striking analogy between animal 

 and vegetable cuticle or skin. In the animal it varies in thick- 

 ness from the delicate film which covers the eye, to the hard 

 skin of the hand or foot, the coarser covering 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, and hard and coarse 

 in the rugged coats of the elm and oak. In the birch you may 

 see the cuticle or outer bark peeling off in circular pieces. 

 This seems not to be endowed with the vital principle, and in 

 this respect it differs from all other parts of the plant. The 

 epidermis or cuticle serves for protection from external inju- 

 ries, and regulates the proportion of absorption and perspira- 

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

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



Proper juices of vegetables Secretions from the cambium Of what are the 

 bark, wood and pith composed ? Divisions of the bark Describe the epidermis. 



