1917] Butters,— Studies in Ferns — Athyrium 173 
side of the stipe. They are usually well separated from each other 
and in a frond broken off at the base, they often project as two per- 
fectly distinct fibrous cords. Taken as a whole, the xylem groups of 
the bundles are somewhat concave on their inner faces, though their 
central portion usually curves in the opposite direction. The tips of 
the xylem groups, and especially those facing the ad-axial side of the 
stipe are always sharply inflexed towards the middle of the stipe. 
These inflexed tips of the xylem may extend in at a sharp angle to the 
main mass of the xylem, in which case they are very conspicuous, 
or they may be completely inflexed, so as to lie closely applied to the 
inner face of the main xylem mass. Milde noted the existence of these 
inflexed parts of the bundle in those ferns in which they are conspicu- 
ous, but he seems to have overlooked the entirely parallel condition 
Fig. 2. Structure of the stipe in the genus Asplenium. A,B, A. Trichomanes L. A, stipe, 
X 10; B, detail of the xylem of A. C-F, A. viride Huds. C, base of stipe, X 10; D, upper 
part of the stipe, X 10; E, detail of the xylem of D; F, rachis of the frond, X 10; G, H, 
A. bulbiferum Forst. G, stipe, X 33. H, rachis of the frond, X 3}. I-K, A. fragrans Sw. 
I, stipe, X 24; J, detail of the xylem of I; K, rachis of the frond, X 5. 
in such species as Athyrium Filix-femina in which the tips of the 
xylem, are so much inflexed as to be almost hidden against the mass 
of the bundle. Sooner or later the two bundles unite by their dorsal 
extremities into a single U-shaped, or rarely V-shaped bundle. The 
structure of the stipe of Dryopteris Thelypteris (L.) A. Gray, D. mon- 
tana (Vogler) Watt and other related species of Dryopteris is identical 
with that of Athyriwm. 
In Asplenium the vascular structure of the stipe is less uniform, 
and Milde recognizes four classes of structure in this genus, besides 
certain anomalous cases (see fig. 2). There are two small, centrally 
located bundles, which may be free throughout the length of the 
stipe and rachis, or may be united above, or even throughout. 
