112 
PHYSIOLOGICAL VIEWS. 
nished with this gas by the decomposition of water* and other sab- 
stances which are absorbed by the root. 
The Cambium is the sap elaborated by the chemical process car- 
ried on in the leaves, and rendered fit for the nourishment of the 
plant. 
In tracing the descent of the cambium or returning sap, we shall 
not find it passing through the same vessels by which it ascended ; 
it is chiefly conveyed by a system of vessels between the liber or in- 
ner layer of the bark, and the alburnum or j^oung wood ; here it 
contributes both to the formation of an outward layer of nev/ wood 
and an inward layer of nev/ bark ; extending also from the extrem- 
ity of the roots, to the upper extremity of "the plant, it furnishes 
materials for the formation of new buds and radicles. 
If a ring is cut through the bark of a tree, the cambium v/ill be 
arrested in its course, and accumulating around the upper edge of 
the bark, will cause a ridge or an annular protuberance. This vege- 
table blood being thus prevented 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 is not made- 
too deep, the v/ound will soon heal by the union of the disconnected 
bark, and the circulation of the cambium proceeds as before. This 
experiment proves the importance of this fluid to the existence of 
the plant. 
The Proper Juices of Vegetables. This division comprehends 
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 system, 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, re- 
sins, gums, essential and fixed oils ; 2d, such as are destined to be 
conveyed out of the plant; these consist chiefly of vapours and 
gases exhaled from flowers, and may, perhaps, more properly be 
called excretions than secretions. 
LECTURE XIX. 
PHYSIOLOGICAL VIEWS — BARK, WOOD, AND PITH — GROWTH OF A DICOTYLEDO- 
NOUS PLANT — GROWTH OF A MONOCOTYLEDONOUS PLANT. 
We have exhibited to your view the minute discoveries made by 
the help of the microscope in the solid parts of the vegetable sub- 
stances ; we have also noticed those important fluids, the circula- 
tion of which appears to constitute the fife, and produce the growth 
of plants. We have now to consider the sohd parts already de- 
scribed, as composing the body of the vegetable, and collected 
under the three forms of Ba.rk^ Wood^ and Pith. 
Bark. The bark consists of the epidermis, cellular integument, and 
cortex. 
1st. Epidermis^ is the skin of the membrane which extends over 
* Water consists of oxygen in union with hydrogen. 
t The word eperdimis is from epi, upon, and derma, the skin. 
Cambium, or descending sap— How conveyed — Importance of this fluid — VvMiat is 
the efl';ct. of cuttinii; a ring through the bark of a tr e ?— What are the proper juices of 
vegetables? — Of what three parts is the body of the vegetable composed 7— Divisions 
of the bark — Describe the epidermis. 
