FLUID PARTS OF VEGETABLES. 
109 
Spiral vessels are so call- 
ed from their form, which 
resembles that of a screw, 
c (Fig. 117, a;) they are 
sometimes termed trachea, 
^ from a supposed analogy to 
^ the trachea of insects, or 
their organs for breathing. 
These vessels are formed 
^ of a thread-like fibre turn- 
ed spirally from right to left. 
Annular vessels, (so call- 
ed from the Latin annulus, 
a ring,) are so perforated 
as to make the tube appear 
to be composed of rings, 
(Fig. 117, b.) 
Moniliform vessels (from monile^ a necklace) resemble, in external 
appearance, a string of beads, (Fig. 117, c;) these serve to connect 
large vessels, and to convey sap from one set to another. 
Mosses, fungi, and lichens, have no vascular system, but their 
tissue IS all of the cellular kind. The solid substance of plants is all 
composed of some varieties of the two kinds of membranes we have 
now desciibed. Roots and stems are made up of vascular fibres; 
these may easily be split longitudinally, as the vessels in this case 
are only separated, and the cellular texture easily yields ; but in 
severing the t oots and stems horizontally, greater resistance is to be 
overcome, since the tubes are to be cut across. 
Vegetables, liKe animals, have a system of glands^ or internal ves- 
sels, which are made subservient to the purpose of producing changes 
in the fluids of the plants; — thus the sap is converted into the proper 
juices ; and from thrt same soil and nourishment plants of very differ- 
ent properties are pioduced. 
Mivbe], by the aid or the microscope, succeeded in discovering a 
system of glands in the pores or cells, and on the borders of the spi- 
ral vessels. There are also external glands, which appear nlanifest 
to the naked eye ; as the nectaries of flowers, which secrete or man- 
ufacture honey ; and the stings of plants, which secrete an acrid sub- 
stance, w^hich, by penetrating the skin, causes a painful sensation. 
Fluid Parts of Vegetables. 
The different fluids which are exhibited in the vegetable body may 
be considered under three general divisions : 1st, the sap, or ascend- 
ing fluid ; 2d, the cambium, or descending fluid ; 3d, 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. Every 
one knows, that if the earth around the roots of plants is destitute of 
moisture, they soon die.^ Water holding in solution various sub- 
stances, such as earths, salts, animal and vegetable matter, is absorb- 
ed by the radicles ; by some unknown process, they 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 
Spiral vessels — Annular — Moniliform — All the solid substance of plants com- 
posed of some of these vessels— The use of glands— Glands discovered by Mirbel— 
External glands — Three kinds of fluids— What is the sap, and how formed?— What 
is the use of the sap-wood 1 
10 
