246 



BOTANY 



are also a number of fossil types of Pteridophytes which are not 

 assignable to any of the three existing classes. 



CLASS I. FILICALES 



The Filicales, or Ferns, are cosmopolitan, but much more abundant 

 in the moist Tropics than elsewhere. In northern regions a few spe- 

 cies like Pteridium aquilinum or Osmunda regalis may be abundant 

 enough to attract notice, but most of the northern Ferns are compar- 



Fio. 211. Spore-division in Eqmsetum. A, I), E, E. telmateia ( X 400) . B, C, 

 E. limosum, more highly magnified. A, group of four sporogenous cells. B, C, 

 first nuclear division; in B, multipolar spindle. D, second nuclear division. 

 E, division of the cell into the young spores. (B, C, after OSTEBHOUT.) 



atively insignificant. In the Tropics, however, especially in the 

 mountains, they often occur in great numbers and variety, and some 

 of the Tree-ferns are among the most striking of all plants. 



The Gametophyte 



The gametophyte (Fig. 212) in most Ferns is a flat, green thallus, 

 which in exceptional cases (e.g. Vittaria sp.~) may reach several centi- 

 metres in length and branch repeatedly. 



The ripe spore usually shows three membranes, the inner cellulose 



