162 



Ml >RPHOLl >GY 



Polypodiaceae. — The Polypodiaceae are leptosporangiate, a peculiar 

 feature belonging to all the Filicales except the Marattiaceae. It means 

 that the sporogenous tissue is developed from the oulcr cell that follows 

 the periclinal division of the superficial initial, instead of from the inner 



>V^^^>. 



Fig. 381. — The cctophloic siphonostele of Osmunda: the central pith is surrounded 

 directly by the xylem (internal phloem having disappeared), which occurs in distinct 

 strands (separated from one another by the pith rays), the mesarch character of which 

 is often very evident; investing the xylem is a thin sheath of phloem; the bundles 

 shown in the cortex are sections of leaf traces. 



cell, as in eusporangiates. The outer cell develops as a papillate-pro- 

 jecting cell (fig. 387), in which three oblique walls appear so as to form 

 an apical cell with three cutting faces. This apical cell cuts off seg- 

 ments to form the elongated stalk. When segment formation ceases, 

 a transverse wall through the apical cell cuts off a cap cell, and leaves 

 a four-sided inner cell completely invested by the three uppermost seg- 

 ments and the cap cell. This centrally placed cell is the primary spo- 



