72 



PLANT LIFE, 



bundles are of two kinds, xylem bundles and phloem bundles, 

 so placed that they alternate with each other about the 

 periphery of the stele (figs. 86, 88, 89). The xylem bundles 

 may be in contact with one another in the center, or the 

 center of the stele may be occupied by a pith (figs. 86, 89). 



Fig. 86. — Transverse section of the stele and a portion of the surrounding cortex of the 

 root of calamus, s, s, innermost layer of cortex, the endodermis, adjoining outermost 

 layer of stele, the pericycle; /, xylem bundles; />/i, phloem bundles. The shaded ele- 

 ments of xylem bundles are the primary xylem ; the large ones, g, are secondary. In 

 the center of the stele and between the bundles is conjunctive tissue. Highly magnified. 

 — After Sachs. 



The tissues of the xylem are usually lignified (see ^[ 9) 

 and, when abundant, make up what is called the wood. They 

 are the chief water-conducting elements of the older parts of 

 the root. 



The tissues of the phloem are usually not lignified, and the 

 most important ones are the sieve- tubes, which conduct proteids 

 from above to the growing regions of the root. 



