THE STEM 123 
pitted tracheary cells are advantageous, as they have greater 
strength than spiral and annular ones. 
Wood fibers. These are long, slender, pointed, dead cells with 
greatly thickened walls and only comparatively few small pits 
(Fig. 117). They serve as strengthening cells. Tracheids that 
have a structure approaching that of wood fibers are called 
fiber tracheids. These 
two types of fibers 
usually make up a 
considerable propor- 
tion of the wood of 
dicotyledonous trees. 
Wood parenchyma. 
Parenchyma cells in 
the xylem are desig- 
nated as wood paren- 
chyma. They serve 
chiefly for the storage 
of food. 
Phloem. The pri- 
mary phloém of the 
dicotyledonous stems 
consists of three types 
of cells: sieve tubes, 
companion cells, and 
phloém parenchyma. 
Sieve tubes. The 
sieve tubes consist of 




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Fig. 120. Cross section of a bicollateral bundle 
of a squash stem, showing phloém on both sides 
of the xylem 
ph, phloém ; c, cambium region ; x, xylem vessel. 
(x 95) 
thin-walled, elongated cells arranged in vertical rows (Figs. 115, 
116). The adjacent cells of a sieve tube are united by small holes 
in the cross walls (Fig. 118). Also, sieve tubes have rounded 
places on the side walls which are pierced by small holes and con- 
nect one sieve tube with another or with a companion cell. The 
areas on the walls of sieve tubes which contain such holes are 
called sieve plates. The sieve tubes get their name from the 
sievelike appearance of these plates. The walls of the sieve 

