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. 



