160 THE ELEMENTS OF VEGETABLE HISTOLOGY 



verse sections cork cells appear rectangular or polyg- 

 onal in outline. In surface view they are irregular 

 in form and, because of their dark color and close 

 contact, rarely present a clear and definite outline. 

 In powdered materials cork cells always appear in sur- 

 face view but, owing to the thickness of the fragments 

 definite details of cell structures are seldom apparent. 



Phellogen. — This tissue, although always present, 

 is not easily distinguished from the cork cells and 

 cortical parenchyma. It occupies a narrow zone 

 immediately beneath the cork. The individual cells 

 are rectangular in sectional view and brownish in color. 

 As this tissue undergoes disintegration during grind- 

 ing, it is never apparent in powdered materials. 



Cortical Parenchyma. — This parenchymatic tissue 

 is similar in every respect to the cells of the 

 primary cortex. Occasionally schlerenchymatic tis- 

 sues or stone cells are developed in the midst of the 

 cortical cells of the root, to which they afford addi- 

 tional support and strength. 



Phloem and Xylem Bundles. — These elements are 

 similar in structure to those already described under 

 Primary Root Tissues. It must be noted that the fibro- 

 vascular bundles of secondary structure are complete 

 and consist of both xylem and phloem tissues. The 

 bundles present in secondary or mature root struc- 

 ture are either of the open collateral type or the 

 bi-collateral type. In the former, the xylem elements 

 are toward the center and are separated from the 

 phloem by a strip or circle of cambium. In the bi-col- 

 lateral bundle the xylem tissue bears cambium layers 

 on its inner and outer surfaces, and each of these in 

 turn gives rise to a phloem group. 



