=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.
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.
The =phloem parenchyma= cells resemble the sieve cells, but they are larger.
The =bast fibres= occur singly or in groups of two or three. The walls are white, non-porous, and non-striated.
=Cambium.= The cambium layer is composed of rectangularly shaped cells, which are frequently obliterated.
=Xylem.= The xylem contains vessels, wood parenchyma, and medullary rays.
The =vessels= are large, rounded cells with thick walls.
The wood parenchyma consists of thick-walled cells of irregular size and form.
The =medullary rays= are tangentially elongated and rectangular in form.
=Pith parenchyma.= The pith parenchyma cells are rounded in outline and as large as the cortical parenchyma cells.
When the roots and rhizomes of spigelia are powdered (Plate 93) they show the following structure:
The epidermal cells are small and brownish on surface view, varying in size from 13 by 18 micromillimeters to 31 by 40 micromillimeters. When associated with parenchyma they appear as black masses. The cortical parenchyma cells are rounded and vary in size from 23 by 26 micromillimeters to 37.5 by 90 micromillimeters. Many of the cells from the root contain larger quantities of minute single rounded starch grains varying in size from 1 micromillimeter to 4 micromillimeters. The larger round single starch grains are found in both the cortical and pith parenchyma of the rhizome. They vary in size from 5 micromillimeters to 18 micromillimeters. The conducting elements are pitted tracheids varying from 10 micromillimeters to 38 micromillimeters in diameter. A few pitted and annular vessels are also found. The only fibres occurring are found in the xylem. They are not a prominent feature of the powder, as their walls break up into minute fragments. The pith parenchyma varies in size from 13 by 19 micromillimeters to 75 by 82.5 micromillimeters. It is in these cells that the largest starch grains occur.