BARK. 
107 
undergone in the parenchyma of the leaves the changes we 
have described, is carried from the leaves into the bark, de» 
Bcending through the tissues, and also flowing laterally along 
the medullary rays into the stem ; or in other words, it is con- 
veyed by a system of vessels between the liber^ or inner layer 
of the bark, and the alburnum^ or young wood ; here it con- 
tributes both to the formation of an outward layer of new 
wood and an inward layer of new bark ; extending also from 
the extremity of the roots to the upper extremity of the plant, it 
furnishes materials for the formation of new buds and radicles. 
If a ring be cut through the bark of a tree, the cambium, or descending juice, will 
be arrested in its course, and accumulating around the upper edge of the bark, will 
cause a ridge or an annular protuberance. This vegetable blood being thus pre- 
vented from having access to the lower part of the plant, the roots cease to grow, 
the sap ascends but feebly, and in two or three years the tree dies. If the incision 
be not made too deep, the wound will soon heal by the union of the disconnected 
bark, and the circulation of the cambium proceed as before. This experiment 
proves the importance of this fluid to the existence of the plant, 
124. Proper Juices. — ^This division comprehends all the fluids 
furnished b}^ the plant, except the sap and cambium, as oils, 
gums, &c. These are the product of the cambium, as, in the 
animal system, tears are secreted from blood. The secretions 
carried on by the vegetable glands from the cambium are ot 
two kinds : 1st, such as are destined to remain in the plant, as 
milk, resins, gums, essential and fixed oils ; 2d, such as are des- 
tined to be conveyed out of the plant ; these consist chiefly of 
vapors and gases exhaled from flowers, and may, perhaps, more 
properly be called excretions than secretions. 
LECTUKE XX. 
PHYSIOLOGICAL VIEWS. BARK, WOOD, AND PITH. GROWTH OF A DI- 
COTYLEDONOUS PLANT. GROWTH OF A MONOCOTYLEDONOUS PLANT. 
125. We have exhibited the minute discoveries made by 
the help of the microscope in the solid parts of the vegetable 
substances, and 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 vegetable, and collect- 
ed under the three forms of Barh^ Wood, and Pith. 
Bark. — ^The hark, in exogenous plants, consists of the epider- 
mis, cellular integument, and cortex. 
Importance of this fluid — Eftect of CDtting a ring through the bark of a tree. — 124. Proper juices.— 
12.1. Division of the solid parts of the plant — Baric 
