PLUID PARTS OF VEGETABLES. 



103 



Fig. 117. 



Spiral vessels are so call- 

 ed from their form, which 

 resembles that of a screw, 

 (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 descnbed. 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 toots and stems horizontally, greater resistance is to be 

 overcome, since the tubes are to be cut across. 



Vegetables, lixe 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 the same soil and nourishment plants of very differ- 

 ent properties are pioduced. 



Mirbel, by the aid or the microscope, succeeded in discovering a 

 sijstem of glands in the pores or cells, and on the borders of the spi- 

 ral vessels. There are also external glands, which appear manifest 

 to the naked eye; as the nectaries of flowers, which secrete or man- 

 ufacture honey ; and the stings of plants, which secrete an acrid sul>- 

 stance, which, 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 7— Wha/ 

 '*the use of the sap-vsrood 1 

 10 



