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MORPHOLOGY 



6. Cyatheaceae (tree ferns). — Annulus vertical and complete (figs. 

 375, 376). About 280 tropical species in 7 genera. 



7. Poly podia ceae. — Annulus vertical and incomplete (figs. 384, 385, 

 300). 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 arc 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 Gleichcnia, showing relation to vein system and absence of 

 indusium (after Diei.s); 368, sporangium of Gleichenia, with equatorial annulus (after 

 Diels); 369, marginal sorus of Hytnenophyllum, showing the stalklike receptacle bear- 

 ing sporangia with an equatorial annulus, and the investing indusium (after Karsten); 

 370, 371, sporangia of Trichoma ncs, 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. 



