60 GROUPS OF TISSUES, OR TISSUE SYSTEMS 



fibrous tissue among the tracheary tissue. In bicol- 

 lateral vascular bundles, the inner mass of phloem is not 

 separated from the xylem by a cambium layer. 



89. Wherever a leaf is attached, one or more vascular 

 bundles in the stem pass out into it. These usually run 

 downward in the stem for some distance before they 

 unite with the other bundles there. In the leaf the 

 phloem portion is downward (i.e. toward the back of the 

 leaf), and the xylem mass uppermost. Here the bun- 

 dles are the so-called "veins." At first 

 they are much like the stem bundles, 

 although usually the cambium is lack- 

 ing, but the more they are divided, the 

 smaller and simpler they become until 

 finally they may consist of only one or 

 two rows of tracheids, a single row of 

 sieve cells, and a row of companion cells, 

 with a few thin-walled parenchyma cells 



surrounding the whole. In some cases these bundles 

 end blindly in the parenchyma of the leaf. In other 

 cases they meet other similar bundles and so form a net- 

 work with no free ends. 



90. Secondary Thickening. The fact that in the for- 

 mation of the open collateral bundles from the pro- 

 cambial strands of meristem tissue, a portion of the 

 meristem remains unchanged as the cambium layer, 

 separating the xylem and phloem, makes it possible for 

 the bundle to continue to grow in thickness. This it 

 does by the growth and periclinal division of the cambium 

 cells, and the transformation of the inner cells thus 

 formed into xylem and of the outer ones into phloem, 

 continually leaving, however, an intermediate portion of 

 cambium which can grow and divide further. 



91. The xylem formed during the process of secondary 



