STEMS 235 
The sieve cells are small, and with thin, white, angled 
walls. 
The phloem parenchyma resembles the sieve cells, but they 
are larger. 
The bast fibres are rounded in outline and the walls are 
thick, white, non-porous, and non-striated. 
Cambium. The cambium cells are rectangular in shape or 
the walls are collapsed and the cells indistinct. 
Xylem. The xylem contains vessels, wood parenchyma, 
medullary rays. The vessels are small and angled, the walls 
are thick and white. 
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 paren- 
chyma, and bast fibres. 
The sieve cells have thin, white, angled walls. 
The phloem parenchyma cells are frequently tangentially 
elongated, otherwise they resemble the sieve cells. 
The bast fibres occur alone or in groups. The walls are 
thick, white and porous. 
