226 HISTOLOGY OF MEDICINAL PLANTS 
usually not clearly seen because the walls are partially 
collapsed. 
Xylem. The xylem is composed of vessels, wood parenchyma, 
medullary rays, and pith parenchyma. 
Vessels. The vessels are slightly angled in outline and few 
in number. 
Wood Parenchyma. The wood parenchyma cells are small 
and angled. 
Medullary Rays. The medullary ray cells are tangentially 
elongated, but in structure resemble the wood parenchyma cells. 
Pith Parenchyma. The pith parenchyma cells are rounded 
in outline and contain small, simple, rounded starch grains. 
CROSS-SECTION RUELLIA RHIZOME 
The cross-section of ruellia rhizome (Plate 92) differs from 
the structure of spigelia rhizome. It is as follows: 
Epidermis. ‘The epidermal cells vary in shape from nearly 
square to oblong, and they are filled with dark-brown cell 
contents. 
Cortex. The cortex contains parenchyma and stone cells. 
The outer layer of the cortical parenchyma cells are variable 
in size and many of the cells contain deposits of calcium car- 
bonate and dark cell contents; the inner parenchyma cells are 
larger and they are free of the dark-brown cell contents, but 
many of the cells contain deposits of calcium carbonate. 
Stone cells with thick, white, porous, and striated walls occur 
in among the cortical parenchyma cells. 
Phloem. The phloem contains sieve cells, phloem, paren- 
chyma, and bast fibres. 
The sieve cells are small and with thin, white, angled walls. 
The phloem parenchyma cells resemble the sieve cells, but 
they are larger. 
The bast fibres occur singly or in groups of two or three. 
The walls are white, non-porous, and non-striated. 
Cambium. The cambium layer is composed of rectangularly 
shaped cells, which are frequently obliterated. 
Xylem. The xylem contains vessels, wood parenchyma, 
and medullary rays. 
The vessels are large, rounded cells with thick walls. 
