XII EQUISETINEAL 471 



divided at once into an inner and an outer cell, the former 

 giving rise directly to the plerome or central cylinder. The 

 next division (seen in longitudinal section) separates the epi- 

 dermis initials from the cortex. A cross-section of the young 

 plerome immediately after the first divisions have taken place 

 (Fig. 275, A) shows that the three primary cells are of unec|ual 

 size, and that the two smaller ones divide first. From the larger 

 one, the first periclinal wall separates a central cell, which occu- 

 pies almost exactly the middle of the section, and this stands 

 immediately above the corresponding one in the older segments, 

 so that in longitudinal sections (Fig. 274) these form a very 

 conspicuous axial row of cells {x, x), which together constitute 



Fig. 275. — Three transverse sections of the young root, X200; tn, endodermis; v, cen- 

 tral vessel. 



the single large vessel which occupies the centre of the older 

 bundle. The endodermis becomes separated by this time, and 

 a little lower down divides by periclinal walls into the two layers 

 found in the completely developed root. The tissues of the cen- 

 tral part of the young root are very regularly disposed (Fig. 

 275, B, C). In the centre is the large vessel already described, 

 around which are arranged at first a single row of usually six 

 or eight cells (Fig. 275, B). By these first divisions the sepa- 

 ration of the xylem and phloem of the bundle is complete. If 

 there are six of these primary cells the bundle will be triarch, if 

 eight, tetrarch. In somewhat older sections of a tetrarch bun- 

 dle (Fig. 275, C) four of the primary cells are still recognis- 

 able and have divided but little. These form the four groups 



