THE STEM. 



I05 



bundles is, however, subject to change. In some cases 

 one of the strands surrounds the other. Commonly it 

 is the bast which surrounds the wood, as in the fernworts 

 (fig. 121). Sometimes independent phloem bundles are 



Fig. 121. — Transverse section of Selaginella, showing three steles, each composed 

 of a xylem bundle surrounded by phloem. /, /, intercellular spaces in cortex, 

 separated from the steles only by the large-celled endodermis. The cells underlying 

 the epidermis are thickened to form mechanical tissue. Magnified 150 diam. — After 

 Sachs. 



found with which are associated no xylem bundles. In 

 the phloem certain cells may develop into fibers, which 

 are not to be confused with the fibers occurring in the 

 pericycle. Some of these, also, are valuable in the textile 

 industries. 



