156 THE ELEMENTS OF VEGETABLE HISTOLOGY 



tions the sieve elements appear as groups of small 

 very thin-walled cells in which the sieve plates or 

 perforated end-walls may occasionally be apparent. 

 Each group of sieve cells is surrounded by bast fibers, 

 which, upon transverse section, appear as thick- 

 walled angled cells with a rather small cavity or lumen. 

 The true character of sieve cells can only be seen 

 in longitudinal sections where they are apparent as 

 long, thin-walled structures showing sieve plates at 

 each end. The bast fibers on longitudinal view 

 appear as long, thick-walled cells tapering toward 

 each end. 



Xylem Bundles. — These tissues are developed in 

 the original plerom parenchyma in a manner similar 

 to those of the phloem bundles. They are usually 

 placed nearer the center of the plerom zone than are 

 the phloem bundles with which they alternate. Each 

 xylem bundle consists of transporting elements, ducts 

 or vessels, and mechanical or supporting elements, 

 wood or xylem fibers. In transverse section the ducts 

 appear as large, irregularly circular, thick ^walled cells, 

 surrounded by smaller, angled, thick-walled wood 

 fiber cells. In longitudinal view the ducts appear as 

 continuous structures, the walls of which show mark- 

 ings varying according to the type of vessel. Wood 

 fibers in longitudinal view appear as thick-walled cells, 

 many times longer than broad and tapering toward 

 each end. 



The arrangement of fibro-vascular bundles in a 

 circle within the endodermis is common to both pri- 

 mary and secondary root structures. The alter- 

 nation of phloem bundles with xylem bundles, each 

 remaining incomplete, is characteristic of the radial 



