INTRODUCTION. 27 
being represented by a few parallel cells obliquely placed near 
the apex of the spore-case, which bursts vertically across the apex 
into two nearly equal and regular parts, or valves, the rupture, 
though extending much further than the parallel cells just men- 
tioned, being no doubt facilitated by their presence. 
The rest of the British Ferns belong to the Ophioglossacew, an 
exannulate group. Here the spore-cases are quite sessile, and open 
«by regular valves. In Ophioglossum there is no spore-case beyond 
the hollowed substance of the contracted spore-bearing leaf. 
In certain of the annulate groups of Ferns, represented by Poly- 
podium itself, the spore-cases spring from the surface of the fronds 
without any perceptible covering; and in these cases the sori are said 
to be naked. In some other groups they are covered while in a 
young state by a membrane of the same form as their own, which 
membrane at length bursts according to its natural habit, and is 
either cast off or pushed aside, as the spore-cases increase in 
size; this membrane, called the Indusium, is very well represented 
in the young fructification of Lastrea and of Asplenium. The 
membranous cover is usually placed above the spore-cases, which is 
the true position of an indusium; but in one or two comparatively 
limited groups of genera, a similar kind of membrane is placed 
beneath the sorus, and continued in an incurved manner, so as to 
invest it; this particular form, which has been called an Involuere, 
is only found among the British genera, in Woodsia. From these 
peculiarities of the cover or indusium the annulate Ferns have been 
subdivided into two lesser groups, called the Indusiate or covered, 
of which the Involucrate is a modification, and the Non-indusiate or 
naked Ferns. 
The spore-cases of Ferns seem to have been long considered as 
special organs, not having any very clear analogy with any part of 
the structure of Flowering plants. Dr. Lindley, however,* has 
* Lindley, Vegetable Kingdom, 75. 

