FERNS AND FERN AEEIES. I9 1 
the indusia are orbicular and flat or flattish. The species 
are divided into two groups, according to the mode of at¬ 
tachment of the indusium. In one group this is attached 
by the center, in the other by a notch or sinus in the side. 
The two methods are well illustrated in the familiar Christ¬ 
mas fern (D. acrostichoides) and the equally common mar¬ 
ginal fern (D. marginalis). It is not unlikely that the gen¬ 
us may eventually be divided and a new genus made of 
those which, like the Christmas fern, have the indusium 
attached by the center. 
In the three remaining genera the sori are arranged lon¬ 
gitudinally in linear or oblong rows and the attachment of 
the indusium is correspondingly different. In Woodward- 
ia, where the sori are sunken in the leaf in interrupted rows 
which resemble chain stitching, the indusium is fixed by 
the outer margin and covers the cavity like a lid. In the 
rare Camptosorus the sori are scattered very irregularly 
and the indusium is membranous. Tastly, in the genus 
Asplenium the indusium is attached to the outer side of the 
sorus and opens on the inner side towards the midrib. The 
common lady-fern (A. filix-fcemina) differs from the other 
species in that the sori are curved so as sometimes to cross 
the fertile veinlet, and in consequence the indusium some¬ 
times appears to open outward rather than inward. It 
sometimes happens, notably in A. acrostichoides (thelypter- 
oides), that some of the sori and indusia are double. In 
that case one half necessarily opens outward. 
It is an interesting question whether the presence or ab¬ 
sence of the indusium denotes the greater degree of speci¬ 
alization. The fact that some of the highest genera are able 
to get along without it would seem to indicate that it is of 
early origin and is now in process of elimination. Possi¬ 
bly the genera with false indusia mark the half-way stage 
of fern economy so far as concerns the protection of the 
young sporangia. 
