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PHARMACEUTICAL BOTANY 



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Fig. io. — Stages in the development of the elements of the xylem. A, progres- 

 sive steps in the develbpnient of a' tracheal tube, i , Row of procambial or cambial 

 cells that are to take part in the fbrma,tion of a tube; 2, the same at a later stage 

 enlarged in all dimensions; 3, the cells in 2 have grown larger, their cfoss^walls 

 have been dissolved out, and' the wall' lias become' thickened and pitted; 4, the 

 walls in 3 have becorhe 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 lube is mature and dead. B, Stages in the formation 

 of tracheids from procamblaJ or cambial cells. The steps are the same as in j4 , 

 excepting that the cross-walls,'remaiii and become pitted. C, steps in the develop- 

 ment of wood fibers from caJnbial cells, x, Cambial cells; 2, the same growth 

 larger in all dimensions with cells shoving past each other as they elongate; 3, a 



