THE STEM 131 
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= * 
Vascular bundles. The vascular bundles of monocotyledonous 
stems are like those of dicotyledonous stems in consisting of 
xylem toward the center of the stele and phloém toward the 
periphery. They differ, however, in not having a cambium 
layer, such as is found in dicotyledons (Fig. 127). This is 






Fie.126. Vas- Fig. 127. Cross section of a vascular bundle of 
cular bundles sugar cane (Saccharum officinarum) 
I t 
ee eo, OU p, parenchyma; s, sclerenchyma of bundle sheath ; 
a, air space; av, annular vessel ; sv, spiral vessel ; 
node of sugar pv, pitted vessel ; st, sieve tube ; sp, sieve plate ; c, 
cane. (X 3) companion cell. (x 185) 
Of an inter- 
connected with the fact that monocotyledons usually do not 
have secondary thickening. Each bundle is generally more or 
less completely surrounded by a sheath of sclerenchyma cells, the 
bundle sheath (Figs. 127, 128), which is particularly well devel- 
oped on the sides toward the center and toward the periphery 
of the stem. The phloém consists mostly of sieve tubes and com- 
panion cells, and the xylem of vessels and wood parenchyma. 
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