168 THE ELEMENTS OF VEGETABLE HISTOLOGY 



than the other and that the end walls are blunt or 

 square. 



Phloem, Cambium and Xylem Tissues. — These 

 tissues are developed as complete fibro- vascular bundles 

 in the plerom zone and thus differ from the corre- 

 sponding bundles in primary roots. In dicotyledonous 

 stems the primary nbro-vascular bundles are of the 

 open collateral type, or rarely the bi-collateral. In 

 monocotyledonous stems the bundles are of the con- 

 centric, or more rarely, the closed collateral type. 

 The cells entering into these different types of fibro- 

 vascular bundles are similar in character to those 

 described in the section dealing with secondary bundles 

 in the root. (Chapter XIII.) 



Pith Parenchyma. — This tissue represents the orig- 

 inal parenchymatic tissue of the plerom zone. The 

 individual cells possess slightly thickened and porous 

 walls, are more or less spherical in form, and show 

 exceedingly large intercellular spaces. The inter- 

 cellular spaces become larger toward the center of 

 the stem; therefore the central pith is a very loose 

 structure, and may be lost or destroyed in the section- 

 ing of herbaceous stems. In monocotyledonous stems 

 the entire region within the endodermis is termed 

 the stele, and consists of a groundwork of pith paren- 

 chyma cells in which are scattered isolated and 

 irregularly arranged concentric or closed collateral 

 bundles. 



CELL CONTENTS OF PRIMARY STEMS 



The cell contents of primary stems are stored 

 in the parenchyma cells of the cortical and pith regions. 

 Although these contents may include a variety of 



