162 
MORPHOLOGY 
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 outer cell that follows 
the periclinal division of the superficial initial, instead of from the inner 
FIG. 381. The ectophloic 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- 
