GENERAL DESCRIPTION. 
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All the British species are circinate (and therefore 
True Ferns) except two — the Adder’s Tongue and 
the Moonwort, in both of which the fronds are what 
is called plicate, or folded straight, like the folding of 
a lady’s fan. 
The order in which the veins, or ribs, of the fronds 
are disposed is called the venation ; and deserves atten- 
tion as affording one of the means of distinguishing the 
groups. It is from some determinate part of the veins 
that the spore-cases proceed. This part is called the 
receptacle. In some few native kinds the receptacle is 
projected beyond the margin of the frond, and the 
spore-cases are collected round its free extremity. 
More commonly, however, the veins stop within the 
margin, and the spore- cases grow in round or elongated 
clusters, situated sometimes at their ends, sometimes 
at their sides, and protruded through the skin of the 
lower surface of the fronds. 
The seeds (it has already been said) are called spores , 
the seed-vessels spore-cases, the clusters of spore-cases 
sort. These sori, generally placed on the back or mar- 
gins of the fronds, are in the great majority of British 
species surrounded or girt by an elastic ring or band, — 
sometimes vertical and burst by an irregular trans- 
verse fissure when the spores, having reached maturity, 
need to be dispersed, — sometimes horizontal or oblique, 
instead of vertical. In the earlier stages of their 
growth the sori are also covered with a thin transpa- 
rent membrane, called an indusium. As the sori grow, 
the indusia get broken and thrust back, sometimes 
flung off. To some species there is no perceptible 
