150 BRITISH FERNS. — SUB-ORDER II. TRIBE II. 
found it in great abundance in 1857, near Stockghyll Force, Amble- 
side, where it clothed a bank of a wet shady lane for six or eight 
yards. In 1865 a similar form was found by the late Mr. J.J. 
Jones and by Mr. J. E. Mapplebeck in the same district. 
6. strictum, Barnes. Found at Witherslack by Mr. J. M. Barnes. 
THE OAK FERN. 
Polypodium Dryopteris.— Linnceus. 
A SMALL, exquisitely beautiful species, inhabiting mountainous, 
rocky situations in shady woods ; or beneath hedges where the 
atmosphere is damp but not the soil. It is sometimes called “ The 
Smooth Three-branched Polypody,” because the frond is three- 
branched. Usually varying in size from 4 to 12 inches, but occa- 
sionally very diminutive. I found it under cavernous stones near 
the summit of Ben Lawers, not much more than one inch long. 
Frond triangular, and of a brilliant refreshing green. Rhizoma 
creeping. This fern can only be confused with the Limestone 
Polypody; but the latter has its fronds as if dusted with lime. It 
is a common Scotch and Welsh fern, and occurs in fifteen English 
counties. In Ireland it is exceedingly rare. Widely spread in 
Europe and America. A rapid-growing fern with a creeping rhizome. 
It is met with in very large clumps. Easily cultivated. 
There are no varieties. 
THE LIMESTONE POLYPODY. 
Polypodium Robertianum. — Hoffmann. 
(P. CALCAREUM, Smith ; and P. DRYOPTERIS var., Bolton.) 
In appearance very like the Oak 
Fern, but more obliquely triangu- 
lar. It varies from 8 to 18 inches 
in length. The stipes is much 
longer than the rachis, whilst in 
Dryopteris it is just the reverse. 
This species delights in sunshine ; 
Dryopteris , in shade. The fronds 
being pubescent, they have the 
appearance of being dusted over 
with lime. Rhizoma, creeping. Found in Cumberland, West- 
moreland, Yorkshire, Derbyshire, Gloucestershire, Wiltshire, 
Somerset, Caernarvon, Denbigh, and Glamorganshire. Near Bux- 
ton its rhizomes creep about heaps of stones ; and plants can be 
obtained by removing the stones, when it is seen that the plants 
are luxuriating without the roots penetrating soil. It is found in 
France, Germany, Switzerland, Hungary, Norway, Canada, the 
United States, and on the Himalayan Mountains. 
There are no varieties. 
F IG. 43. — Polypodium Robertianum 
(pinna, showing fructification). 
