THE FERNS 



135 



st 



the "annulus," a row of cells with thickened walls 

 whose contraction plays an important part in the open- 

 ing of the ripe sporangium and the discharge of the 

 spores (Fig. 35, G, 

 H,r). 



On comparing 

 the Eusporangiatse 

 and Leptosporan- 

 giatee, one is at 

 once struck by the 

 great disparity in 

 the numbers of the 

 two groups. Prob- 

 ably all living 

 species of Euspo- 

 rangiatse, including 

 the peculiar genus 

 Isoetes, whose posi- 



sorus covered with "the "kidney-shaped In- 



tion here is by no 

 means certain, 

 scarcely exceed one 

 hundred, while the 

 Leptosporangiates, 

 the typical ferns, 

 number probably 

 at least 3500 to 



FIG. 35 (Leptosporangiate Ferns). A, leaflet 

 of a shield-fern (Aspidium), showing the 

 sori, or sporangial groups, s ; B, a single 

 sorus covered with the kidney-shaped in- 

 dusium, in; C, a filmy-fern (Trichomanes) 

 with the sorus surrounded by a trumpet- 

 shaped indusium ; D, longitudinal section 

 of the sorus, showing the sporangia borne 

 upon the elongated columella; E, F, young 

 sporangia of Poli/podium falcatum seen in 

 section ; the sporogenous cell is shaded ; 

 t, t, the tapetal cells which later are broken 

 down; G, a ripe sporangium of the same 

 species showing the ring or annulus, r, and 

 the stomium, st, where the opening occurs ; 

 H, sporangium of a climbing fern (Lygo- 

 dium) with terminal annulus, r. 



4000 species. In 



spite of this extraordinary difference in numbers of 

 species, the former group shows much greater range 

 of structure, so much so that it is necessary to make 

 two and perhaps three orders to include them, and the 

 relationships of these are very doubtful. The Lepto- 



