BRITISH FERN'S. 3 
under side, while at intervals from the upper spring 
leaves, which, when young, are very pretty objects, 
being curled up in a kind of scroll, that gradually 
unrolls as they rise upward. The bodies which repre- 
sent the seeds here (called spores) are usually produced 
in formations growing upon the backs of the leaves, and 
it is principally upon the mode of arrangement of these 
formations (called sori) that the classification of Ferns 
is founded. 
" The common condition of the apparatus in which 
the spores are produced may be described as follows : 
On the backs of the leaves, round patches, or streaks, 
or lines running round the borders of the divisions, 
appear, which in a perfect state have a brown, powdery 
aspect. This appearance is concealed in many kinds, 
in the early stages, by a membranous cover enclosing 
the brown dust ; when the spores are more advanced, 
these coverings (called indusia) become either wholly 
or partly detached, and if examined with a magnifying 
glass, are found to have peculiar forms in different kinds 
of Ferns, and to be attached sometimes by little stalks, 
and sometimes hy their edges, if we place some of the 
brown dust-like substance under a microscope, we find 
it to consist of a number of little cases, which, when 
ripe, burst, and discharge the very minute spores which 
have been produced within them. The bursting of the 
cases results from the elasticity of a kind of thickened 
band (the annulus), which extends around the mem- 
branous case, or spore-fruit (tlieca). The spores are 
mostly so small as to be invisible singly to the naked 
