PLANT TISSUES 145 
A PRosTELE is a primitive form of stele in which the vascular 
tissues form a solid cylinder, the phloem surrounding the xylem. 
It occurs in young parts of stems of club-mosses, in the stems of 
some ferns and in the roots of secondary growth of most plants. 
(See Fig. 83A.) 
A SIPHONOSTELE resembles the prostele except that a pith 
occurs in its centre. There are two kinds of siphonosteles, viz., 
the ectophloic in which the phloem is external to the xylem, as 
(a) ; 3 » 
| 
A B Cc 
Fic. 83.—Diagrams showing the types of arrangement of vascular tissues in 
steles. A, protostele. B, siphonostele. C, dictyostele. (From “An Introduction to 
Plant Anatomy” by Eames and Mac Daniels, McGraw-Hill Book Co., Inc. Publishers.) 
in the stems of gymnosperms and angiosperms, and (2) the 
amphiphloic in which there is an external phloem outside the 
xylem and an internal phloem on the inner face of the xylem, 
as in the stems of some ferns and certain families of dicotyledons. 
(See Fig. 83B.) 
A DicryosTELe is a siphonostele whose vascular mass is 
broken up into a number of longitudinal strands or vascular 
bundles. This type is common to most monocotyl stems, the 
bundles being sometimes called meristeles. (See Fig. 83C.) 
BraNcH TRACES.—These are strands of vascular tissue which 
arise from the vascular bundles of the stele of the stem and run 
out into the lateral branches. Usually two traces or bundles 
arise for each branch, but sometimes only one arises. When 
