120 
BRITISH FERNS. 
5. Polyp odium Robertiaimm* Hoffm. Rigid 
Three-branched Polypody. 
Sori like the last. Caudex creeping. Stem firm, beset with 
glands. Frond triangular. 
The name of this somewhat doubtful species is derived 
from that of the Geranium Robertianum, or Herb Robert, 
whose scent that contained in the glands of the fern re- 
sembles. It was called formerly Polypodium calcareum , 
that name referring to the limestone districts, the inva- 
riable habitat of the fern. 
The caudex of the Polypodium Robertianum creeps 
widely, and in this respect, as well as in the sori, it is 
not distinguishable from the Oak-fern ; indeed, so great 
is the similarity between the two, that many high au- 
thorities consider this merely a variety of the former 
species, and have figured the one for the other. Hoff- 
mann lays principal stress on the glandular down cover- 
ing the fern as a mark of distinction, this feature being 
wholly absent in the Oak-fern. Sir W. Hooker, though 
deciding that “ Polypodium Robertianum is a very doubt- 
ful species/* yet concedes that its habit is very different to 
that of P. Dryopteris . The stems are much firmer, less 
deflexed at the juncture with the frond, and different 
in colour, being of a dull green instead of tinged with 
brown or purple ; the branches are more upright, over- 
lapping one another so as to avoid the divergence of the 
form of the frond towards a pentagon. The colour of 
the frond is quite different from that of the Oak -fern, 
being dull in tint, as if a grey misty light were shed 
over it, while the other glowed in sunshine. This dul- 
ness of colour is in part ascribable to the secreted glands 
which cover the fern as with down, but the actual colour 
