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204 FERNS OF NORTH AMERICA.
SO that the pinnæ are pod-like, and either sub-cylindrical or
somewhat moniliform. The venation is free, and the sori are
dorsal on the veins. Mr. Faxon writes: “ The indusium can
be detected only when the fertile frond is very young, and
appears as a very delicate, lacerate membrane, attached at the
base of the receptacle, and serving to separate the sorus from
its neighbors. I have not found it in any case hood-like as
in 0. sensibilis. The sori are quickly confluent, and all trace
of the indusium is soon lost. The membranaceous edge of
the transformed fertile pinna is attached near the bases of the
inferior sori and a fold is usually found pressed against the
sori as seen in the drawing (Fig. 3). This is usually ruptured,
so as to leave a portion attached at the base of the sorus,
and must not be mistaken for the true indusium, which is
within.”
The sporangia have twenty-six or twenty-eight articulations
of the ring. The spores are dark-colored and ovoid.
Imperfectly fertile fronds are often found, which are analogous
to the “ obiusilobata" condition of O. sensibilis.
Plate L X X I I I ,— Onoclea Struthiopteris, about half the naturak size
of a small plant. Fig. 2 is part of a fertile pinna with one side laid
open to show the venation and the sori. Fig. 3 is a cross section of
the same, showing on one side a sorus with its indusium, and on the
other a denuded receptacle, the indusium showing in section only. Fig.
4, an indusium. Fig. 5, part of a sterile pinna. Fig. 6, a section of
the stalk. Fig. 7, a spore.
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