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 parench^Tna. 



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 parenchj^nia 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 parench\aTia 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 parench^Tna 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 parench^-ma, 

 and medullary rays. 



The vessels are large, rounded cells with thick walls. 



