62 BAST FIBRES. [CH. IV 



represents what is known as a pitted cell wall ; the de- 

 pressions are the pits, and the thin layer, which separates 

 pit from pit, is the pit-membrane. 



The dots seen on the walls of the large xylem vessels 

 are pits of a slightly more complicated kind than that 

 shown in fig. 26. The spiral lines and rings which 

 distinguish the narrow vessels at the central side of 

 the bundle are the result of still more complicated 

 thickening. 



In the xylem the non-vascular elements are the cells 

 forming the parenchyma of the xylem, and the xylem- 

 fibres (fig. 23 x.f\ elongated, tapering, thick-walled cells. 

 In the phloem attention must also be called to certain 

 non- vascular elements, minute elongated cells, called 

 companion cells, whose chief interest from our present 

 point of view is that they give a characteristic aspect 

 to phloem in transverse section and help us to learn 

 to recognize this tissue. The phloem also contains some 

 simple cellular elements making up the phloem parenchyma t 

 which is of no special importance. 



Fig. 24 shows in longitudinal section the pericycle 

 fibres to which reference has already been made. They 

 are narrow elongated elements with very small cavities 

 and thick, lignified walls, lying directly internal to the 

 endodermis. They form a tough resisting tissue possess- 

 ing in fact, to some extent, the quality that gives a 

 commercial value to the fibres of the flax, hemp and 

 other plants. 



There remain to be considered the cambium and the 

 cortex. 



