
in secondary thickening continues 
throughout the life of the plant. 
It is in this way that the trunks of 
trees continue to grow in diameter. 
After the cells which are to form 
xylem or phloém are cut off from 
the cambium, they undergo one or 
more tangential divisions before be- 
pent ee pay 
yal me 
: 
1 

ing differentiated into permanent 
tissue. In this way the cambium 
frequently comes to be bordered on 
both sides by cells which are very 
similar in appearance to cambium 
cells. The result is that on super- 
ficial observation the cambium ap- 
_ pears to be several cells thick. 
Bast fibers. In the secondary 
phloém there is very frequently an 
alternation of bands of thick-walled 
sclerenchyma fibers (known as bast 
fibers) and bands of thin-walled 
phloém cells (Figs. 139, 140). The 
bast fibers are like other scleren- 
chyma cells in being elongated, 
pointed, thick-walled dead cells 
whose function is to give strength 
to the group of tissues in which 
they occur. The term Cast fiber is 
frequently used in a collective sense 
to denote the sclerenchyma fibers 
of the cortex, the pericycle, and the 
phloém. Strands of bast fibers have 
long been used by man for indus- 
trial purposes, having furnished 
early savages with bowstrings and 
material for cloth. Among the 
THE STEM 



iV /\ 
i ' 

= 
se Se c 
Teo 
a6 22 =. 
Fic. 141. Cross section of the 
secondary wood of a linden stem 
Rows of cells extending from top 
to bottom, pith rays; large, thick- 
walled openings, vessels; smaller 
empty cells, wood fibers ; small cells 
with gray contents, wood paren- 
chyma. An annual ring ends with 
the small cells in the lower part 
of the drawing; another begins 
with the large vessels and ends near 
the top, just below the large vessels 
of the succeeding ring. (x 255) 
