1917] Butters,— Studies in Ferns — Athyrium 183 
The scales of A. angustum are quite different from those of either 
of the other species just mentioned. They are of moderate size (up 
to 1 em. long and 1.5 mm. wide, usually considerably shorter than this). 
In shape they are narrowly linear-lanceolate with a fairly wide base, 
and in falling they leave larger and more conspicuous scars than do 
the wider scales of A. Filix-femina. They are much more opaque 
than those of the last mentioned species, and generally of darker color, 
varying from the “Mars Brown” of Ridgway’s Color Standards, 
Pl. 15, to nearly black, with the middle often darker than the edges. 
Under a moderately strong hand-lens it is difficult to make out any 
structure, while under a compound microscope, they are seen to be 
composed of narrow fibrous cells about 15 times as long as they are 
wide. 
In the form of the frond, the two American ferns are decidedly dis- 
similar. A. angustum closely resembles A. Filiz-femina, but its stipes 
are commonly proportionally longer,— often one-half as long as the 
fronds,— and the lower pinnae are not quite so much reduced in size, 
and are less strongly deflexed than in that species. 
A. asplenioides has still longer stipes, about equalling the narrowly 
deltoid lanceolate fronds. The second pair of pinnae are commonly 
the longest, and the basal pinnae are only very slightly reduced in 
length. 
The details of the fronds of both American species are, as in A. Filiz- 
femina, very variable, but the following points are worthy of note: 
the fronds of A. angustum are often markedly dimorphic, the segments 
of the fertile fronds being much narrower and more acute than those 
of the sterile fronds; the pinnules of the fertile fronds of A. angustum 
are commonly narrowly lanceolate and acute, those of A. asplenioides, 
oblong or linear-oblong and obtuse (in very large fronds, however, 
the pinnules may be deltoid-lanceolate and sub-acute, and the seg- 
ments of the third order oblong and obtuse.! 
In the structure of the sori and indusia, the two east American 
species are markedly different from A. Filix-femina and from each 
other. The sori in both American species are prevailingly asplenioid 
and for the most part over 1 mm. in length, those of A. asplenzor 
being longer than those of A. angustum. Athyrioid sori are generally 
1 For a discussion of the various forms and varieties of A. asplenioides and A. angustum, see 
below, p. 188 et seq. For illustrations of the pinaules of the various forms of these species. 
see Plate 123, figures 3-18. 
