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 
fis. 28. Ending 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 
es am — ait 
