12 BRITISH FERNS. 



minate portions of the veins, which are thickened and form 

 the receptacles. Each separate mass or cluster of the seed- 

 cases is called a soitis, but being generally spoken of collec- 

 tively, the plural term son' becomes much more frequently 

 used. The sori are marginal when they project beyond the 

 margin, and dorsal when occupying some part of the under 

 surface of the frond. 



The seed-cases called also spore-cases, or sporangia, or 

 ihecce are mostly minute roundish-oval bodies, containing 

 one cavity, and nearly surrounded by a jointed vertical band 

 called a ring, which is continued from the base so as to form 

 a short stalk, by which they are attached. When they have 

 reached maturity, the elasticity of the ring bursts the case 

 by an irregular transverse fissure, and the seeds or spores, in 

 the shape of fine dust, which is almost invisible, become dis- 

 persed. This occurs in the majority of the native species. 

 In Trickomanes and Hymenophyllum, however, the elastic 

 band is horizontal or oblique, not vertical ; and in Osmunda, 

 Botrycltium and Ophioalossum, the spore-cases are two-valved, 

 and destitute of the elastic ring. 



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, but eventually, by their 

 growth, becoming disrupted at its margins and thrust back, 

 or not unfrequently cast off before the maturity of the seeds. 

 Some species, however, never bear any visible indusium, 

 even in their earliest stages. The presence or absence of an 

 indusium, or cover to the seed-heaps, is consequently one of 

 the technical points by which 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 

 HymenophyUums. 



Taking now a retrospective glance, we have seen that the 

 Ferns are, as regards external structure, flowerless plants, 

 having erect or creeping stems, which bear the leaf-like 

 fronds ; and on some part of the surface of the fronds, usually 

 the lower side, but sometimes the margin, are borne the 

 clusters of seeds, which, in the majority of the native species, 



