COLLATERAL BUNDLES 59 



The innermost vessel borders a large intercellular air 

 space. Partly enclosed between the large pitted vessels, 

 but in the main placed peripherally to the xylem, is 

 the phloem. In cross-section this is elliptical and 

 consists of large sieve tubes and small companion cells. 

 The whole bundle is surrounded by a mass of cells, mainly 

 fibrous 1 . No meristem tissue is present at all in the com- 

 pleted bundle. 



87. Open collateral vascular bundles can be found 

 most typically in the class Dicotyledonese, though they are 

 also present in the Strobilophyta and related groups. 

 In the stem they are usually placed almost equidistant 

 from the center, surrounding a central mass of paren- 

 chyma, the pith, and separated from each 



other laterally by the masses of paren- 

 chyma (primary medullary rays), which 

 connect the pith to the cortex. The ten- 

 dency to anastomose is very great in open 

 collateral bundles, so that these medullary 

 rays are interrupted above and below at 

 frequent intervals, and are not continuous 

 for a long distance in the stem. Bicollateral bundles of 

 the open collateral type are similarly placed in the stem. 



88. When first completed, the xylem portion consists 

 of two or three to several rows of tracheary tissue, usually 

 not crowded but loosely placed with reference to each 

 other, and with the spaces filled in with parenchyma. 

 The outer boundary of the xylem is parallel to the 

 surface of the stem, and is succeeded by a layer, one to 

 several cells thick, of meristem, the so-called cambium. 

 Bounding this externally is the phloem region, consisting 

 at first of sieve and companion cells and other par- 

 enchyma tissue, and sometimes even of masses of bast 

 fibers. In young woody stems there may be considerable 



