66 



THE PLANT CELL. 



be closed at times, owing to the forcing up of the torus against either 

 of them. 



The development of bordered-pits may be observed in longitudmal 

 sections of the young stem. The traoheides which have been just formed 

 by the cambium show thin circular wall-areas, round which the first 

 indication of the border soon appears (see Fig. 50). 



(«) Tracheides or vessels with annular or spiral thickenings 

 are to be seen in the protoxylem of Dico- 

 tyledons and Coniferse, and also in the xylem 

 of fibro-vascular bundles of Monocotyledons 

 and Ferns (see Fig. 43). The endings of the 

 leaf-bundles also show these elements. 



(/) Wood-fibres are also seen at times, 



t 



Fig.46.— A Wood Fibre. 

 — p, Small pits ; these 

 are slit-like, with a 

 thickened border. 



Fig. 47. —Details op Xylem Elements (Piniu). 

 — Tracheides with bordered-pits (p) on the 

 radial walls, tt, tracheides; I, lumen of 

 pit; c, tangential wall; ,-, reticulations. 



