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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. 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 Dicotyledonex, 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 ,,. 576 = 
rays are interrupted above and below at collateral vas- 
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 
