ROOTS AND RHIZOMES PAPA 
Pericycle. The cells forming the pericycle are sieve cells 
and phloem parenchyma. ‘The sieve cells are small, angled cells 
with extremely thin, white walls. 
The phloem parenchyma resemble the sieve cells, except 
that they are larger. 
Cambium. The cambium cells are rectangular in shape; 
the walls are thin and white. 
Xylem. The xylem is composed of tracheids, wood paren- 
chyma, and wood fibres. 
Tracheids. ‘The tracheids are the largest diameter cells of 
the centre of the root. The walls are thick and the cells are 
slightly angled in outline. 
Wood Parenchyma. The wood parenchyma cells surrounding 
the tracheids are five to seven, angled, and the walls are not. 
so thick as the walls of the tracheids. 
Medullary Rays. The medullary ray cells resemble the 
structure of the wood parenchyma cells, but they are radially 
elongated. 
Pith Parenchyma. ‘The cells forming the pith parenchyma 
are larger than the cells of wood parenchyma, but their struc- 
ture is similar. 
CROSS-SECTION RUELLIA ROOT 
The cross-section of ruellia root (Plate 90) shows the follow- 
ing structure. It should be carefully noted how the structure 
differs from that of pink root: 
Epidermis. The epidermal cells are angled and variable in 
size; many of the epidermal cells are modified as root hairs. 
Hypodermis. The cells of the hypodermis are one layer 
in thickness and their structure is similar to the epidermal 
cells. 
Cortex. The cortex contains parenchyma and stone cells. 
The outer layers of the cortical parenchyma cells are round in 
outline, and they contain dark-brown cell contents, while the 
cortical parenchyma cells bordering on the endodermis are small 
and they are free of dark-brown contents. 
Many of the inner parenchyma cells contain amorphous 
deposits of calcium carbonate. 
