CONIFERALES 



247 



cell and at its next division, the outer cell becomes a phloem cell, and 

 the process is repeated. However, there is no such regular alterna- 

 tion, and the embryonic region may be more than one cell thick, as 

 shown by mitotic figures. At the beginning, there must be only one 

 cambium cell for each row of cells, as seen in transverse section ; be- 



FiG. 254. — Picea nigra: transverse section of seven-year-old stem, showing xylem, 

 cambium, and phloem. The phloem, except that of the last year, is crushed, and the 

 cells have lost all their contents; X433. 



cause each row of xylem cells is continuous with a row of phloem 

 cells (fig. 253). 



After the first year, the cells of the phloem lose their contents and 

 become variously crushed and crowded (fig. 254). Later, they dis- 

 appear entirely, weathering off, together with the original cortex, so 

 that the comparative amounts of xylem and phloem, as shown in 

 fig. 253, are maintained only for a short time. 



