THE HARD FERN. 209 
Blechnum on the other hand is known by the indusium springing 
directly from the under surface of the frond, the margin extending 
beyond it. This is a clear and intelligible difference, and the genera 
are only satisfactorily divided when these peculiarities are allowed 
to have full force. 
In many or indeed most of the exotic species of Blechnum, the 
difference is obvious, on account of the fertile fronds not being 
narrowed; but there are other species in which these fronds are 
contracted as in Lomaria, but in which at the same time the sori are 
not strictly marginal and the indusium is clearly a development from 
the under surface and not from the edge of the fronds. These 
latter, among which the British species is included, ought undoubt- 
edly to be retained in the genus Blechnum, the accident of their 
fronds being contracted alone causing the apparent but unreal 
resemblance to the structure of Lomaria. 
Viewed in this light, Blechnum includes two sections containing 
species having some difference of aspect. The $ Eublechnum repre- 
sented by the B. occidentale of South America, and B. orientale of 
the East, produces the sori close or nearly elose to the costa, a con- 
siderable space intervening between'them.and the margin of the 
frond. The $ Parablechnum, which includes the British B. Spicant, 
and the South African B. punctulatum, has the sori also near the 
costa, but it becomes submarginal by the contraction of the fronds, 
and the loss of nearly all the space exterior to the indusium which 
is so obvious in the other group. "There are a considerable number 
of species of Blechnum, and these are dispersed widely p in the 
Old and New World. 
In nearly all the plants having the blechnoid type of fructification, 
the veins of the fronds are free, that is, distinct at their apices, but 
there are a few exotie species which present modifications of the 
reticulated forms of venation. Two of these have been long known, 
namely Salpichlena in which the venules are combined at the 
margin, and Sadleria in which the veins are arcuately combined near 
the costa. Another form, the Blechnum melanopus of Sir W. J. 
Hooker, a native of the Khasya Hills, has recently been described 
and figured;* in this the veins are more decidedly reticulated, 
* Hooker, Species Filicum, iii. 64, t. 161. 
VOL. II. 

