138 PHARMACEUTICAL BOTANY 
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Fic. 76.—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 overhanging 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 are the same as in A, excepting 
that the cross-walls remain and become pitted. C, steps in the development of 
wood fibers from cambial cells. 1, Cambial cells; 2, the same growth larger in all 
dimensions with cells shoving past each other as they elongate; 3, a later stage 
