PHYSIOLOGICAL VIEWS. 
ander side of leaves ; they contain a fluid, at first transparent, 
but which becomes white, yellow, or of a reddish hue. These 
vessels are long, branched tubes, which anastomose freely, and 
thus resemble the vessels of the animal system. 
a. The solid substance of plants is all composed of some varieties of cellular and 
vascular tissue. Roots and stems are made up of vascular fibers ; these may easily 
be split longitudinally, as the vessels in this case are only separated, and the celhi- 
lar texture easily yields ; but in severing the roots and stems horizontally, greater 
resistance is to be overcome, since the tubes are to be cut across. 
b. Vegetables, like animals, have a system of glands, or internal vessels, which are 
made subservient to the purpose of producing clianges in the fluids of the plants ;— 
thus the sap is converted into the proper juices ; and from the same soil and nour- 
ishment plants of very different properties are produced. Mirbel, by the aid of the 
microscope, succeeded in discovering a system of glands in pores or cells, and on the 
borders of the spiral vessels. There are also external glands, as the nectarifer 
ous glands in the corollas of flowers and the stings of plants. 
FLUID PAKTS OF VEGETABLES. 
119. The different elementary fluids in the vegetable body 
may be considered under three general divisions : the swp^ or 
ascending fluid j the cambium, ov descending juice / the proper 
juices. 
The sap is a limpid, inodorous liquid, the elements of which 
are imbibed from the earth by pores in the radicles of the 
root. Water, holding in solution various substances, such as 
earths, salts, animal and vegetable matter, is absorbed by the 
radicles, which by some unknown process convert this fluid 
matter into sap ; and then, by means of vessels which form 
what is called the sap-wood, or alburnum, this sap ascends 
through the stems to the branches ; passing through the woody 
part of the petioles, and those minute branches of the petiole 
which form the ribs and veins of the leaf, it enters into the ves- 
sels and cells which extend throughout its substance. In the 
leaves the sap undergoes changes in composition, after which it 
is returned to the bark. 
a. The ascending sap is always in circulation, but its energy varies with the sea 
sou and the age of the plant. Heat has an important influence in quickening the 
ascent of the sap ; yet, during a dry and hot season, it often appears to ascend but 
slowly. This is because the absorption of fluids from the earth is checked by the 
dryness of the soil. The plant, by a little stretch of the imagination, may be con- 
sidered as thirsty, and thus man may seem not only provident, but humane, in ad- 
ministering to its roots refreshing draughts of water. Even the leaves, at such a 
period, seem too impatient to wait for supplies by means of the connecting sap- 
vessels ; for if water be sprinkled upon them, they fail not to use their own power 
of absorption, and upon such an application, may be seen to revive almost instanta 
neously. When the moisture of the earth coincides with elevation of temperature, 
the sap ascends with the greatest rapidity ; this is the case in spring. It is at this 
period that incisions are made into the wood of maple-trees, in order to procure sap 
for the manufacture of sugar. The sap may at this time be seen flowing almost in 
a. All the solid substance of plants comp')sed of tissue— 6. Glands.— 119. Three kinds of fluids 
What is the sap, and how formed 1 — Wiiat is the use of the sap-wood ? — a. Eftect of drought u\a 
the plant— Why are incisions made in ma jle-jees in the spring, rather than at any othe perit«l 3 
