THE STRUCTURE OE PERNS. 21 



the elastic band is horizontal or oblique, instead of verti- 

 cal ; and in Osmiinda, BotrycJiiuni, and Ophloglossum, the 

 spore-cases are tu"o-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 mojority of those which are natives of 

 Great Britain, the sori are covered in the earlier stages of 

 growth by what is commonly called the vidusinm. 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 

 bach, or frequently even cast off before the maturity of the 

 seeds. Some species, hoM^ever, never bear any visible 

 indusiura, 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 

 Hym en ophyllums. 



