ROOTS AND RHIZOMES 2238 
The stone cells are porous and striated, and the walls are 
thick and white. 
Endodermis. The endodermal cells are tangentially elon- 
gated, 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 gr), 
and by ruellia rhizome (Plate 92). 
CROSS-SECTION SPIGELIA RHIZOME 
The cross-section of spigelia rhizome (Plate gr) 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. 
Cambium. The cambium cells are rectangular, and they are 
