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THE STEM 121 
of a broken ring (Fig. 113). Each of these groups is a vascular 
bundle. A vascular bundle of a dicotyledonous stem consists 
of three parts (Figs. 115-116). The region nearest the center 
of the stem contains thick-walled cells and is known as xylem. 
The peripheral portion of the bundle is composed of thin-walled 
cells called phloém. The xylem and phloém are separated by a 
cambium layer, which is com- 
posed of meristematic cells. 
By division the cambium layer 
increases the size of bundles 
by forming xylem cells on the 
inner side and phloém cells on 
the outer side. 
Xylem. The xylem which 
is formed before the activity 
of the cambium has begun to 
produce xylem and phloém 
cells is called primary xylem. 
This primary xylem is com- Fre. 118. A sieve plate of a squash 
posed of two parts. The xylem stem (Cucurbita maxima) pierced by 
formed first is nearest the in- holes, walls of sieve tube, and sur- 
terior of the stem and is rounding cells. (x 110) 
called protorylem. The more 
peripheral part of the primary xylem is known as metaxylem. 
Xylem is composed of three different types of cells: namely, 
tracheary cells, which include tracheids and vessels; wood fibers ; 

and wood parenchyma. 
Tracheary elements. The tracheids are elongated dead cells, 
with walls that are thick in some places and thin in others. 
They serve to conduct water, or both as water-conducting and 
as strengthening cells. The thin places in the walls allow 
water to pass readily from cell to cell, while the thickened por- 
tions protect the cells from being crushed by the pressure of 
the surrounding tissues. As the tracheids are dead cells and 
without protoplasmic contents, they cannot develop turgidity, 
and so, if the walls were uniformly thin, the pressure of the 
