6io 



THE STRUCTURE OF ECONQMIC PLANTS 



phloem. The secondary tissues appear in transection as four large 

 wedge-shaped sectors that are separated from one another by 

 prominent rays formed by the continued activity of the pericyclic 

 cells. (Fig. 317.) These may be interpreted as pericyclic rather 

 than secondary xylem rays, since the cambium is confined to the 

 four sectors of vascular tissue. As secondary thickening proceeds, 

 the cortical and epidermal tissues are split; but the pericyclic 

 region continues to keep pace with cambial activity, maintaining 

 itself as a multi-layered zone outside of the secondary phloem 



and producing a phellogen which 

 forms the protective periderm. 



The secondary xylem consists 

 of large vessels, tracheids, fibers, 

 and small isodiametric paren- 

 chymatous cells which surround 

 the vessels and are connected with 

 them by simple or half-bordered 

 pits. The short vessel segments 

 are very wide, in some cases being 

 0.5 mm. in diameter; and in the 

 ontogeny of the vessel, the end 

 walls of a series of segments are 

 digested so as to form a continu- 

 ous trachea. (Fig. 10.) Tyloses 

 occur frequently in the older 

 vessels as a result of the intrusion of bladder-like projections of 

 adjacent parenchymatous cells through the pits. These sac-like 

 protrusions contain cytoplasm and usually a nucleus, and the intro- 

 duction of several such processes, together with their further 

 division, frequently results in the complete filling of the vessel 

 with tyloses that become flattened by reciprocal pressure. (Fig. 

 318.) 



The large size attained by the vessels throws them out of a regular 

 radial alignment, so that they appear as scattered units distributed 

 through each of the xylem wedges. The fibers are long and slender 

 with tapering ends and their number and arrangement may vary. 

 In some cases, the secondary xylem consists largely of fibers and 

 tracheids surrounding the vessels; in others, the fibers occur 

 only in small groups; but in most instances, each vessel is nearly 

 enclosed by tracheids which are separated from it by a zone of 



Fig. 317. Diagrammatic transection of 

 mature root : ca, cambium ; />c/, pericycle ; 

 fcl ra, pericyclic ray ; pd, periderm ; ph, 

 phloem; xy i, primary xylem strand; xy 

 1, secondary xylem. 



