40 



DIFFERENTIATION OF THE TISSUES 



The cells composing the tracheal tubes soon die, so that the 

 tube is not long alive after it comes to maturity. The wood 

 fibers may live but one or only a few years, while the tracheids 

 in respect of length of life seem to vary between the tracheal tubes 



Fig. 19. — stages in the development of the elements of -the xylem. A, progressive 

 steps in the development of a tracheal tube. 1 , row of procambial or cambial cells that are 

 to take part in the formation of a tube; z, 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, 



