Ih e indusium is a plane, or vaulted, or cup-shaped membrane,
produced irom the receptacles of each soriis, called special, and
generally falls off (deciduous,) as it becomes replicate. When
produced from the centre of the receptacle, it is c entral;
when on one side, lateral; and when its base is attached all
rou n d the base or margin, calyciform. Wh en central it is usually
orbicular, with its margin depressed and free all round, or it
is inflated. When lateral, it is either reniform, oval, or oblong;
and attached by a point or base of one side, or linear, and its
whole length fastened on the one side of an elongated receptacle,
the other side being free. W h en calyciform, it is entire
and globose; b u t after a time it opens with an entire or lacin-
iated margin, forming a cup which contains the sporangia.
F re q u en tly all the margin or lobules of the frond are changed
in tex tu re , forming an accessory indusium, with which the
interior lateral attached special indusium more or less connives;
united, they form a vertical or reflexed, continuous or urceolate
calyciform, or bilabiate marginal cyst, which contains the spo-
rangia, and opens exteriorly. Sometimes the whole of the sori
of each segment are enclosed within a universal indusium,
which is formed by the re volute margin of fertile contracted
fronds.
E . J . LOW E .
OhserEcitory, Beeston, M a y \5 th ., 1855.
PERNS;
BRITISH AND EXOTIC.
POLYPODIACE^. R . B r o w n .
T h is very interesting division of F erns have the sporangia,
which are more or less transparent, of an oval or round form.
The sporangia are all unilocular and pedicellate. Sometimes,
h u t very seldom, they are sessile.
The opening of the seed vessels runs transversely, and the
sporangia in the greatest number, are supplied with an elastic
ring, which surrounds them vertically, and is articulated,
(jointed.) In a few examples, this ring instead of being
vertical, is found to be oblique.
TRIBE I.
G YM NOGKAM MA. D e sv a u x .
T h is tribe takes its name from the circumstance that the
linear sori are not furnished with an indusium, and are th e re fore
naked. The derivation of the word Oiymmyramma being
from the Greek gymnos—naked, and gramma—writing.
The veins in the frond are branched, and their apices are