THE STRUCTURE OE FERNS. 
21 
the elastic band is horizontal or oblique, instead of verti- 
cal ; and in Osmunda, Botrycliium^ and Opliioglossum^ the 
spore-cases are two-valved, and either destitute of the 
elastic ring, or having it reduced to a very rudimentary 
condition. 
In a considerable proportion of the known species of 
Ferns, and in the majority of those which are natives of 
Great Britain, the sori are covered in the earlier stages of 
growth by what is commonly called the indusium. This 
is mostly a thin transparent membranous scale of the same 
general form as the sorus itself, at first completely covering 
or enclosing the young seed-cases. Eventually, however, 
by their growth, its margins are disrupted, and it is thrust 
back, or frequently even cast off before the maturity of the 
seeds. Some species, however, never bear any visible 
indusium, and its presence or absence is consequently one 
of the technical points by which the large body of Ferns 
are divided into groups of moderate extent. 
In some Ferns the indusium, or cover, or at least what 
is considered analogous to it, is cup-shaped, containing the 
seed-cases ; but this form is of very rare occurrence among 
the native species, and exists only in Trichomanes and the 
Hymenophyllums. 
