1. Epidermis. 2. Cortical parenchyma. 2´. Intercellular space. 3. Endodermis. 4. Pericycle. 5. Cambium. 6. Xylem. 7. Pith.] =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 parenchyma, 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 structure is similar. CROSS-SECTION RUELLIA ROOT The cross-section of ruellia root (Plate 90) shows the following 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. [Illustration: PLATE 90 RUELLIA ROOT (_Ruellia ciliosa_, Pursh.). 1. Epidermis with root hair. 2. Parenchyma cells with dark contents. 3. Sclerid. 4. Parenchyma without dark cell contents. 5. Endodermis. 6. Bast fibers and phloem. 7. Cambium. 8. Xylem. 10. Pith.] The =stone cells= are porous and striated, and the walls are thick and white. =Endodermis.= The endodermal cells are tangentially elongated, and the walls are thin and white. =Pericycle.= The cells forming the pericycle are the sieve cells, bast fibres, and phloem parenchyma. The =sieve cells= are small, angled cells with thin, white 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. They are rounded in outline, and the walls are white, non-porous, and non-striated. =Xylem.= The xylem is composed of vessels, wood parenchyma, and wood fibres. =Vessels.= The vessels are rounded in outline and few in number.