THE ALPINE POLYPODY. 79 
occasional, or even rare, and they appear never to occur in company 
with the more perfect sori, but only where the spore-cases are much 
fewer in number than usual. To us they have the appearance of 
lacerated membranaceo-filamentous expansions of those points of the 
veins which form the receptacles; and appear to arise from some , 
abnormal condition, perhaps inherent, which limits the power of 
producing spore-cases to the side or base of the receptacle, while on 
the upper side its cells are directly prolonged into the indusioid 
membrane. In no ease have we seen what could be considered as 
a perfect and fully developed indusium. On the other hand, Mr. 
Rylands, of Warrington, who regards the plant as an Athyrium, has 
communicated the result of some observations made in 1855, in 
company with Mr. Wilson, as follows :—“ In those sori which were 
large and fully ripe, the indusium could not be seen, though I 
imagine dissection would have shown traces of it. One sorus was 
found still closed, the spore-cases little developed; it was reniform, 
and lay alongside the venule. In many of the smaller sori remains 
of an indusium was seen, and in two or three it was as nearly 
perfect as one may expect to find it. The margin was laciniated 
with fine projecting points. The laciniated margins are produced 
by the rupture of the cuticle, and the fine points are the cell-walls 
thereof. The indusium is very tender, shrivels, and where the 
spore-cases are numerous, is speedily concealed or perhaps displaced. 
by them ; it is smaller than in the other forms of Athyrium. These 
peculiarities seem to result from the rupture of the cuticle taking 
-place early in the progress of development of the sori; but that it 
has the true indusium of an Athyrium I think cannot be further 
disputed.” Subsequently, in reply to a suggestion that the sup- 
posed indusium in these plants was not like the indusium of a true 
Athyrium, and was not developed at all where the sori seemed 
most perfect, Mr. Rylands wrote :—“ The ‘indusia’ of alpestre 
are not, I think, confined to the imperfect sori, though after 
bursting they soon shrivel and disappear in the larger ones. I have 
compared it with A. Filix-femina molle, and though in texture, 
position, and general character, there was little difference, I am 
compelled to admit that while in the case of alpestre the spore-cases 
seemed to lie within the proper cuticle of the frond, the evidence of 

