PTERIDOPHYTES 



125 



(food-conducting cells), 

 such an arrangement be- 

 ing called the concentric 

 arrangement. This most 

 primitive vascular system 

 of the stem, therefore, 

 consists of a single, solid 

 cylinder with concentric 

 xylem and phloem (com- 

 monly spoken of as a 

 concentric cylinder), to 

 which type the name 

 protostele has been given. 

 In mature stems of Lyco- 

 poditim, the solid xylem 

 strand may divide into 

 branches which run 

 through the stele as 

 several strands sepa- 

 rated by pithlike tissue 



Figs. 268-270. — Sporangium of Lyco- 

 podium: 268, section showing a young 

 sporophyl! bearing a superlicial initial cell 

 (one of a transverse row) on its adaxial 

 face near the base; 269, further develop- 

 ment of the initial; 270, division of initial 

 Into primary wall cell (outer) and primary 

 sporogenous cell (inner). — After Bower. 



Fig. 267. — Cross section of the central region of an 

 adult stem of Lycopodium, showing the inner region of 

 the cortex surrounding the central stele, in which the 

 branches of the xylem mass are irregularly distributed ; 

 section of four leaf traces are seen in the cortex, each of 

 them showing the simple concentric arrangement (xylem 

 surrounded by phloem). 



(fig. 267). From the vascular cyl- 

 inder strands pass out through the 

 cortex, where they are called leaf 

 traces (fig. 267), and enter the 

 leaves, where they become continu- 

 ous with the veins. 



Sporangium. — The large spo- 

 rangium is borne upon the upper 

 (adaxial) surface of the sporophyll, 

 near the base. The sporangium 

 initial is superficial (fig. 268), and 

 is a transverse row of six to twelve 

 cells; in some cases it consists of 

 two or three such rows. Each of 

 these initial cells divides by a peri- 

 clinal wall (parallel with the sur- 

 face), resulting in an outer and an 

 inner transverse row of cells (figs. 



