ROOTS AND RHIZOMES 223 



The stone cells are porous and striated, and the walls are 

 tliick and white. 



Endodermis. The endodermal cells are tangentially elon- 

 gated, and the walls are thin and white. 



Pericycle. The cells forming the perlcycle 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 outUne, 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 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. 



The phloem parenchyma cells resemble the sieve cells, but 

 they are larger. 



Cambium. The cambium cells are rectangular, and they arc 



