Pig. 51. —Stages in the development of the elements of the xylem. A, pro¬ 
gressive steps in the development of a tracheal tube. 1, 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 pferome or cambial cells. The steps 
