XIII 



EQUISETINEA£ 



449 



giving rise directly to the plcrome. The next division (seen in 

 longitudinal section) separates the epidermis initials from the 

 cortex. A cross-section of the young plerome immediately 

 after the first divisions have taken place (Fig. 236, A) shows 

 that the three primary cells are of unequal size, and that the 

 two smaller ones divide first. From the larger one, the first 

 periclinal wall separates a central cell, which occupies almost 

 exactly the middle of the section, and this stands immediately 

 above the corresponding one in the older segments, so 

 that in longitudinal sections these form a very conspicuous 

 axial row of cells {x, x), which together constitute the single 

 large vessel which occupies the centre of the older bundle. 



/ \ 



Fig. 236. — Three transverse sections of the young root, X 200 ; e7i, endodermis ; v, central vessel. 



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 central 

 part of the young root are very regularly disposed (Fig. 236, 

 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. 236, B). By these first divisions the separa- 

 tion 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 

 bundle (Fig. 236, C) four of the primary cells are still recognis- 

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



2 G 



