=Wood Parenchyma.= The wood parenchyma cells are variable in size and shape, but all the cells are angled in outline. =Medullary Rays.= The medullary ray cells are not clearly distinguishable. =Pith Parenchyma.= The pith parenchyma cells of the centre of the root resemble the cortical parenchyma cells. That the structure of rhizomes is similar to the structure of roots is shown by the drawings of spigelia rhizome (Plate 91), and by ruellia rhizome (Plate 92). CROSS-SECTION SPIGELIA RHIZOME The cross-section of spigelia rhizome (Plate 91) is as follows: =Epidermis.= The epidermal cells are nearly angled and free of cell contents. =Cortex.= The cortical parenchyma cells are usually slightly tangentially elongated. The cells of the outer layers are larger than the cells of the inner layers. =Phloem.= The phloem contains sieve cells 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. [Illustration: PLATE 91 CROSS-SECTION OF RHIZOME OF SPIGELIA MARYLANDICA, L. 1. Epidermis. 2. Cortical parenchyma. 3. Phloem. 4. Cambium. 5. Xylem. 6. Internal phloem. 7. Pith with starch.] [Illustration: PLATE 92 CROSS-SECTION OF RHIZOME OF RUELLIA CILIOSA, Pursh. 1. Epidermis. 2. Cystolith. 3. Stone cell. 4. Cortical parenchyma. 5. Bast fibres. 6. Pericycle. 7. Xylem. 8. Pith.] =Cambium.= The cambium cells are rectangular, and they are 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 carbonate 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, parenchyma, and bast fibres. The =sieve cells= are small and with thin, white, angled walls.