126 A TEXTBOOK OF GENERAL BOTANY 
Pith rays. ‘The vascular bundles are separated from each other 
by radial rows of parenchyma cells (Figs. 113, 116) known as 
pith rays.!. The pith-ray cells are usually elongated in a radial 
direction. They serve primarily for the conduction of food and 
water radially in the stem and for the storage of food. Food 
material which is stored 
in the wood parenchyma 
is conducted to these 
cells by the pith rays. 
The pith rays also serve 
for the conduction of 
water from the xylem 
toward the other parts 
of the stem. If a pith 
ray from an old stem is 
stained with iodine, it 
will usually be found to 
contain numerous starch 
erains. 
Pericycle. The region 
between the vascular 
bundles and the cortex 
is known as the pericy- 
cle. It is usually com- 
Fig. 128. Latex tube of poinsettia (Euphor- posed of parenchymaand 
Ina pulcherrima). (xX 125) sclerenchyma cells, but 
the sclerenchyma cells 
may be absent. The sclerenchyma may occur as separate patches 
(Figs. 114, 116) or as a continuous ring in the outer part of the 
pericycle (Fig. 113), forming a sharp line of demarcation between 
the stele and the cortex. The sclerenchyma cells in the pericyele 

1 The term wood ray is proposed in place of pith ray by some modern anato- 
mists, on the ground that these rays do not belong to the pith, but that the 
parts of the rays between the primary xylem represent tracheids which have 
been transformed into parenchyma cells. Owing to the widespread use of the 
term pith ray in literature it seems convenient to retain it, even though, from 
the standpoint of origin, it may be inappropriate. 


" 
© 
ee ee 
ye op ph ewe 
ee 
edie 
| 
ee. cae eee 
ee 
s 
3 
7 
i: 
2 
<= 
Fi 

