18 HISTORY OF BRITISH FERNS. 



bursts the case irregularly, and the seeds or spores, in the 

 shape of fine dust, almost invisible, become dispersed. 

 This is what occurs in the majority of the native species ; in 

 Trichomanes and the Hymenopliyllums, however, the elastic 

 band is horizontal or oblique ; and in Osmunda, Botrycliiurn, 

 and OpJiioylossum, the spore-cases are two-valved, and des- 

 titute of the elastic ring. 



In a considerable proportion of the known species of 

 Perns, 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 indiwum, which 

 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 cast off, 

 frequently even before the maturity of the seeds. Some 

 species, however, never bear any indusium, and its presence 

 or absence is consequently one of the technical points by 

 which the large body of Terns are divided into groups of 

 manageable extent. In some Perns 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. 



