THE FERNS 135 
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 Eusporangiate 
and Leptosporan- 
giate, one is at 
once struck by the 
great disparity in 
the numbers of the 
two groups. Prob- 
ably all living 
species of Euspo- 
rangiate, including 
the peculiar genus gig. 35 (Leptosporangiate Ferns).— A, leaflet 
* 2 of a shield-fern (Aspidium), showing the 
Isoétes, whose posi sori, or sporangial groups, s; B, a Hu le 
tion here is by no sorus covered with the kidney-shaped in- 
x dusium, in; C, a filmy-fern (Trichomanes) 
means certain, with the sorus surrounded by a trumpet- 
shaped indusium; D, longitudinal section 
scarcely exceed one of the Borne, EhOMine ie eporanela borne 
i . upon the elongated columella; E, F, youn 
hundred, while the sporangia ot Polypodium Jalcatum een a 
5 section; the sporogenous cell is shaded; 
Leptosporangiates, t, ¢, the tapetal cells which later are broken 
the typical ferns down; G, a ripe sporangium of the same 
species showing the ring or annulus, 7, and 
number probably the stomium, st, where the opening occurs ; 
H, sporangium of a climbing fern (Lygo- 
at least 8500 to dium) with terminal annulus, r. 
4000 species. In 
spite of this extraordinary difference in numbers of. 
spécies, 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- 
