PTERIDOPHYTA 



265 



last lateral segment of the apical cell. The inner and radial walls 

 of the annulus cells become very much thickened, and when the 

 ripe sporangium dries, the strong contraction of these cells acts like 

 a spring stretched over the apex of the sporangium, and pulls it 

 open at the stomium, the split extending far back through the 

 lateral cells of the wall. The annulus bends far back and then 

 returns to its original position with a quick jerk, which throws the 

 spores for a long distance. 



FIG. 229. Botrychium Virginianum. A, B, germinating spore (X 600). C, sporo- 

 phyte attached to the gametophyte, pr (x 2). D, section of gametophyte, showing 

 the foot of the young sporophyte. E, young leaf ( X 2) . 



Classification of Filicineae 



The Filicinese may be divided into two subclasses Eusporan- 

 giatse and Leptosporangiatae. 



SUBCLASS I. EUSPORANGIAT^E 



The Eusporangiatse comprise but a small proportion of existing 

 Ferns, and show many evidences of being the most primitive mem- 

 bers of the class. This is evinced both by the characters of the 

 gametophyte and of the sporophyte. The three orders included 



