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MORPHOLOGY 
6. Cyatheaceae (tree ferns). Annulus vertical and complete (figs. 
375, 376). About 280 tropical species in 7 genera. 
7. Polypodiaceae. Annulus vertical and incomplete (figs. 384, 385, 
390). About 2000 species in 109 genera, including most of the ferns of 
ordinary observation in temperate regions. 
It must not be supposed that the characters given above are the 
only ones used in separating the families. There are much more funda- 
FlGS. 366-371. Types of annulus: 366, sporangium of Osmunda, with a group of 
thick-walled cells representing the annulus, and a vertical line of dehiscence (after 
LUERSSEN); 367, sori of Gleichenia, showing relation to vein system and absence of 
indusium (after DIELS); 368, sporangium of Gleichenia, with equatorial annulus (after 
DIELS); 369, marginal sorus of Hymenophyllum, showing the stalklike receptacle bear- 
ing sporangia with an equatorial annulus, and the investing indusium (after KARSTEN); 
370, 371, sporangia of Trichomanes, with equatorial annulus (after SADEBECK). 
mental ones that will appear as the groups are considered. The se- 
quence of the families as given represents the general relationship, 
the Marattiaceae being recognized as the most primitive in struc- 
ture, while the Polypodiaceae are the most specialized and the most 
recent. 
Sporophyte. The sporophyte is characterized by a subterranean 
stem, except in the tree ferns, climbers, and epiphytes. In Marattiaceae 
this stem is short, tuberous, and radial, and covered by the persistent leaf 
bases (fig. 377). In the other families it is dorsi ventral and more or less 
elongated (fig. 382), a habit apparently acquired by modern ferns, and 
derived from the older radial type. 
