


234 THE BRITISH FERNS. 
among the British Ferns, by the continuous lines of marginal sori, 
produced on compound fronds which are not contracted. It is the 
Filiz femina of the. older writers, though not of Linneus. Like 
all other widely diffused species it is subject to variation in outline 
and division, as well as in vestiture, but the variations are of little 
importance. 
The nature of the membrane which is found lying beneath the 
sori, in the fully fructified fronds, and which we have called an 
inner indusium, has at various times received the attention of 
botanists ; but with little result as to its actual value in a systematic 
point of view. It appears to have been first made known by the 
late Mr. T. Smith, in a letter communieated to Sir W. J. Hooker, 
in 1819, and published in the Flora Scotica, (p. 2,156). Mr. Smith 
regarded this inner membrane as the true indusium or involucre. 
He describes it as being formed exactly opposite to that which is 
seen on the edge of the frond, the line of capsules or spore-cases 
being placed between the two. “It may be called the inner invo- 
luere, and much resembles the outer, having like that a ciliated 
edge, but instead of being flat it curls inwards covering the capsules 
in their young state, and being itself covered by the outer one; it is 
best perhaps seen when the capsules are about half ripe, at which 
time it is nearly the same breadth as the outer one, and is readily 
seen by the assistance of a microscope. I have found it," he 
continues, “in P. caudata, which is very nearly allied to aquilina; 
it also occurs in P. esculenta, and our mutual friend Brown autho- 
rises me to say, that it is found in a small group of the genus Pteris, 
the species of which agree in habit, and are mostly extra-tropical." 
Sir J. E. Smith has recorded the same structure; and Mr. Newman 
subsequently described it with illustrative figures. More recently 
Mr. W. Wilson has published some excellent drawings* of the 
double indusium found in P. aquilina, esculenta, and scalaris. It 
would seem that the lines of spore-cases lie between two membranes 
naturally projected outwards from the receptacle, which structure 
does not very widely differ from that which occurs among the 
Lindsee ; but in this case the revolution of the margin brings the 
smaller or inner of these membranes into contact with the under 
* Hooker, Species Filicum, iii. t. 141. 

