
THE STEM 149 
the cambium cells, which are at the center of the bundle, cease 
to form xylem and phloém but give rise to pith-ray cells. In this 
way new pith rays are formed in the bundles (Figs. 140, 143-145), 
As the secondary pith rays do not extend to the central pith or 
to the outer edge of the 
phloém, they are not 
as long as the primary 
rays. When the sec- 
ondary thickening has 
progressed farther, ter- 
tiary pith rays make 
their appearance. They 
are naturally not as 
long as the secondary 
rays. In the same way, 
as thickening proceeds, 
other and progressively 
shorter pith rays are 
formed as is shown in 
Figs. 144 and 145. 
The pith rays extend 
radially in the stem for 
considerable distances, 
the primary rays reach- 
ing from the pith into 
the bark; but the ver- 
tical extension is shght. 
The radial extension of 
the pith rays is of ad- 
@sesenci 
pee 
Le) ie ;. 
(2) 
GE 

j Oo 2 
! al ° (2 i 
Fic. 146. Cross section of wood of coral tree 
(Erythrina fusca), a tree that has an unusually 
large amount of wood parenchyma and in 
which the wood parenchyma and wood fibers 
are exceptionally distinct 
The rows of cells containing starch grains are pith 
rays, the small thick-walled cells are wood fibers, 
the small thin-walled cells between them are 
wood parenchyma, and the large openings are 
vessels. (x 85) 
vantage, as they conduct food materials and water radially in 
the stem. A considerable vertical extension would offer no such 
advantage but would result in weak places where the stem 
could be rather easily split. The vertical extension of pith 
rays is seen very readily in tangential sections. Tangential sec- 
tions are longitudinal sections perpendicular to the pith rays 
(Fig. 147). 

