134 
BARK. 
ried 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 vapors and gasses exhaled fr. in flowers, and 
are called excretions. 
LECTURE XVIII. 
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 vege- 
table substances ; and we have also noticed those important 
fluids, the circulation of which appears to constitute the life, and 
produce the growth of [plants. We have now to consider the 
solid parts already described as composing the body of the vege- 
table, and collected under the three forms of Bark, Wood, and 
Pith. 
BARK. 
The bark consists of the epidermis, cellular integument 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 va- 
ries in thickness from the delicate film which covers the 
eye, to the hard skin of the hand or foot, the coarser cov- 
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, and hard and course 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 outer bark 
serves for protection from external injuries, and regulates 
the proportion of absorption and perspiration through its 
pores. It is transparent as well as porous, so as to admit to 
the cellular iutegument, the free access of light and air, 
while it excludes every substance which would be injurious. 
Bark, Wood and Pith — Epidermis or cuticle — 
