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.
=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).
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.
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.]
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.
