PHARMACEUTICAL BOTANY 



FIG. 51. Stages in the development of the elements of the xylem. A, pro- 

 gressive steps in the development of a tracheal tube, i, Row of plerome or 

 cambial cells that are to take part in the formation of a tube; 2, the same at a 

 later stage enlarged in all dimensions; 3, the cells in 2 have grown larger, their 

 cross- walls have been dissolved out, and the wall has become thickened and 

 pitted; 4, the walls in 3 have become more thickened, the pits have an overhang- 

 ing border, the walls have become lignified as indicated by the stippling, and 

 finally the protoplasts have disappeared, and the tube is mature and dead. B, 

 Stages in the formation of tracheids from plerome or cambial cells. The steps 



