Wood parenchyma. The cells are variable in size and shape, and the walls are thick. The medullary ray cells are small, narrow, and tangentially elongated.
Internal Phloem. External to the pith parenchyma are isolated groups of internal phloem consisting of sieve cells.
Pith Parenchyma. The pith parenchyma cells are oval in form and irregularly placed. The cells contain small, simple starch grains.
RUELLIA STEM
The cross-section of ruellia stem (Plate 96) is as follows:
Epidermis. The epidermal cells are variable in shape and very large. There are no cell contents.
Cortex. The cortex consists of collenchyma and parenchyma cells and stone cells.
The collenchyma cells have very small, angled cavities and very thick walls. These cells make up the greater part of the cortex.
The cortical parenchyma cells are variable in size and shape. The stone cells occur singly or in groups. The walls are thick, white, porous, and striated, and the central cavity is frequently quite large.
Phloem. The phloem contains sieve cells, phloem parenchyma, and bast fibres.