54 
FERXS : BRITISH ATvD FOREIGN. 
the true indusia, which in these cases are always attached on the 
interior side of the receptacles, and the two combined indusia form 
■continuous or interrupted grooves, or urceolate, bilabiate, or tubulose 
cysts, open exteriorly and containing the sporangia (plate I. figs. 
10 and 11). 
Universal indusia occur in cases when the segments of the fertile 
fronds are contracted. They consist simply of the margins of the 
segments being more or less changed in texture, and rolled inwards 
so as to include all the sori upon the segment (plate I. fig. 12). 
There is also another kind of indusium, called “ indusoid scales 
they only occur in a few species of the division Eremobrya. In 
Pleopeltis this consists of orbicular, peltate, glistening imbricate 
disks, covering the sporangia ; in Hymenolcpis they are very thin 
and membranous ; in Schellolepis they are very irregular in form, 
and seem to be imperfect sporangia ; their deformity being caused 
by the excessively crowded immersed sporangia ; they are also found 
in Tcenitis and Vittaria, and have received the name of paraphyses. 
I however do not use this term in describing those genera. The 
orbicular disks of Pleopeltis, however, seem to be more special 
■organs, particularly in the smooth-fronded species. 
I have now explained tlie terms of tlie chief organs 
and structure of Ferns made use of for their classifi- 
cation. I fear a beginner will say it is quite enough to 
deter any one from entering upon the study of F eras ; 
but he should bear in mind that it is quite as impossible 
to read a language without first learning the alphabet 
as to understand botanical descriptions without first 
mastering the technical terms employed in them. 
He will be further impressed with the difficulty of 
study when he finds that the very first point of inves- 
tigation is to determine whether the fern before him 
has or has not a ring to its spore-cases. He presumes 
that a microscope is required to determine this first 
starting-point ; but such is not actually the case, for 
with the aid of a pocket lens he will be able to detect 
the presence or absence of a ring, and as annulate 
