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).
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.