60 ELEMENTS OF PLANT ANATOMY. 
furnish examples of this arrangement. These bundles of sieve- 
tubes also occur outside the ring in the rind of the Cucurbitaceae 
family, and in species of Potamogeton a small bundle of this 
tissue is enclosed in thick-walled cells. Bast cells and sieve- 
tubes are both elements of the compound bundle and will be 
described later. 
COMPOUND OR FIBRO-VASCULAR BUNDLE. 
The origin of the bundles at the stem end of the plant has 
already been described, and it has been explained that the cam- 
bium cells may originate partly from the periblem, partly from 
the plerome. All compound bundles originate in a similar manner, 
and all agree in developing from their cambium two classes of 
tissue which are called phloem and xylem. In the type previously 
described, the bundle of the dicotyledonous stem, the phloem is 
developed toward the circumference, the xylem toward the 
centre. Each of these contains four distinct kinds of cells or 
elements. 
Circumference. 
Sieve-tubes. 
Accompanying cells.! 
Phloem. : 
Parenchymatiec cells. 
Bast. 
Cambium. 
( Tracheae or ducts. 
3 Tracheids. 
Xylem. ; 
Parenchymatic cells. 
Libriform. 
Centre. 
All these elements are not necessary to the formation of a 
compound bundle, as the bast and libriform may be wanting. 
There are different kinds of compound bundles according to the 
1 These cells are generally called cambiform, owing to their resemblance to 
cambium, 
