ROOTS AND RHIZOMES 221 



Pericycle. The cells forming the pericycle are sieve cells 

 and phloem parench^Tna. 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 epidennal 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. 



