PTERIDOPHYTA 



225 



Fig. 181. Early development of the sporangium oi Lye o podium selago. A, radial section 

 of base of young sporophyll (l) arising from stem (st), showing initial cell (one of a transverse 

 row); B, slightly later stage; C, division of initial into primary wall cell and primary 

 sporogenous cell (latter shaded); D, tangential section of same; E, further development of 

 sporogenous tissue; F, radial section of same; G, later stage, showing development of 

 tapetum; H, sporangium showing stalk, wall, tapetum, and spore mother cells. {After 

 Bower.) 



dichotomously branched stems showing the radial type of stele, while 

 many of the cone-bearing species have stems with monopodial branching 

 and most of them show the parallel-banded type of stele. 



The large solitary sporangium is always adaxial in its relation to the 

 sporophyll and is also unilocular. The sporangium arises from a super- 

 ficial group of initials consisting of a transverse row of 6 to 12 cells (Fig. 

 181 A, B). In some species there may be two or three such rows. Each 



