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A TEXTBOOK OF GENERAL BOTANY 
neighboring cells would cause them to collapse. The walls of 
the tracheids are heavily impregnated with lignin. This material 
serves to strengthen them without greatly interfering with 

ae 34 | fi 
iB ae | eS 
Fie. 119. Sclerenchyma fi- 
bers from a leaf of a cen- 
tury plant (Agave cantula). 
(x 300) 

their permeability. 
Vessels are composed of rows of 
tracheary cells the cavities of which 
are connected by the total or partial 
disappearance of the cross walls. The 
diameter of vessels is usually much 
greater than that of tracheids, and, as 
they form long tubes, they constitute 
the principal water-conducting ele- 
ments of the dicotyledonous stem. 
Tracheary cells may be divided into 
several types according to the method 
by which the walls are thickened 
(Figs. 115, 116). Annular tracheary 
cells have thickenings in the form of 
rings, while spzral tracheary cells have 
spiral thickenings. Pitted tracheary 
cells have walls which are uniformly 
thickened except for thin places in 
the form of pits. When the pits are 
long and narrow and lie horizontally 
one above the other, the tracheid or 
vessel is said to be scalariform, owing 
to the ladderlike appearance of the 
thickenings. . 
The protoxylem is composed largely 
of annular and spiral vessels and paren- 
chyma, while the tracheary elements 
of the secondary xylem are pitted. Protoxylem is differentiated 
while the stem is elongating, and so the spiral and annular thick- 
enings of the vessels are of special advantage, as they allow the 
vessels to be stretched. The metaxylem and secondary xylem 
are formed after increase in length has ceased, and therefore 

