PLANT ORGANS AND ORGANISMS 115 
several smaller ones in the xylem portion of the bundle which faces 
toward the center of the section. In the outer or phloem portion 
of the bundle will be seen an area of soft small-celled sieve tubes and 
phloem parenchyme. The entire bundle is enclosed by a several 
Fig. 53. — Photomicrograph of a representative portion of Greenbrier stem 
showing epidermis ( e.p .), cortex (c), endodermis ( e.n.d .). cylinder sheath ( c.s .), 
sclerenchyme fibers of closed collateral bundle ( b ), fundamental parenchyma 
( f.p .), trachea (/). X 22. 
layered ring of sclerenchyme fibers, which on the inner face are 
called wood fibers, on the outer, bast fibers. The wood fibers con- 
stitute the supporting elements of the xylem, while the bast fibers 
are the supporting elements of the phloem. 
