STEMS 



35 



procambium strands. These three regions are best shown 

 in a cross-section (Fig. 15). In a little older portion 

 of the stem, such as shown in a section further back 

 (Fig. 15), further differentiation 

 has taken place, which changes 

 involve the ground meristem and 

 the procambium. The vascular 

 bundle is composed of three re- 

 gions : phloem, cambium and xylem. 

 The center of the stem is made up 

 of large, loosely fitting cells which 

 constitute the pith or medulla. 

 Radiating from the medulla out 

 between the vascular bundles are 

 a number of cells which make up 

 the medullary ray. 



Dicot Vascular Bundle. — De- 

 tailed examination of a dicot vas- 

 cular bundle in cross- and longitu- 

 dinal sections shows each of its 

 three parts to be made up of 

 characteristic structural elements. 



Phloem. — In the phloem are 

 sieve tubes, companion cells, and 

 phloem parenchyma. Each sieve 

 tube is a single cell, much elon- 

 gated and modified for conduc- 

 tion. The end walls of sieve tubes 

 (Fig. 16) are thickened, and per- 

 forated by a great number of holes, 



and thus resemble a sieve. Each sieve tube is adjoined by 

 a single row of small cells, the companion cells, which run 

 parallel to it. Phloem parenchyma cells are somewhat 



F ' "^^ 



Fig. 16. — Vascular elements. 

 A, annular tracheal tube; B, 

 spiral tracheal tube; C, reticu- 

 lated tracheal tube; D, pitted 

 tracheal tube; E, cross-section 

 through plate of sieve tube, 

 and adjoining companion cell; 

 F, lengthwise section of sieve 

 tube; G, portions of two com- 

 panion cells. (£, F, and G 

 after Strashurger.) 



