GENERAL DESCRIPTION. 
9 
being circinate. 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 sori. These sori, generally placed on the 
back or margins 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 transverse 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 transparent membrane, called an 
indusium. As the sori grow, the indusia get broken 
and thrust back, sometimes flung off. To some 
