144. A TEXTBOOK OF GENERAL BOTANY 




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Fig.140. A vascular bundle in a cross sec- 
tion of a portion of a young stem of a tropical 
tree (Muntingia calabura) which has undergone 
some secondary thickening 
The xylem is composed largely of vessels and 
wood fibers. In the phloém region there is an 
alternation of bands of thick-walled bast fibers 
and thin-walled phloém cells. A secondary pith 
ray is shown in the center of the bundle. (x 70) 
the cell that is differ- 
entiated is next to the 
xylem, it forms xylem, 
while if it 1s next to 
the phloém, it becomes 
phloém. In this way 
new cells are added — 
to the xylem and the 
phloém, and the bun- 
dles increase in size 
(Figs. 159, 140). While: 
there is more or less al- 
ternation in the pro- 
duction of xylem and 
phloém cells from a 
cambium cell, more 
cells are formed on the 
xylem side than on the 
phloém side. g 
The cells formed 
from the cambium in 
the region of the pith 
rays become  pith-ray 
cells. The activity of 
the cambium thus in- 
creases the length of the 
pith rays, so that the 
bundles and pith rays 
grow equally. 
The formation of new 
cells from the cambium 
results in an enlarge- 
ment of the stem that 
is known as the second- 
ary thickening. The 
formation of new cells 

