OAK FERN . 
91 
rhizoma. By care and attention to these facts, this 
beautiful little fern will be found a pleasing and satis¬ 
factory addition to a fernery. From its small size, it 
is well suited for a fern case. 
VARIETY. 
POLYPODIUM CALCAREUM, or the Limestone Poly¬ 
pody (known also as Poly podium Robertiana ), appears 
to be merely a variety of the Oak Polypody, of 
stouter growth, and occurring in limestone districts. 
Its chief distinctions consist in the pinnate rather 
than the ternate divisions of the fronds, which have a 
glandular, mealy, or tubescent appearance. In de¬ 
velopment, the fronds never assume the appearance 
of the three little balls, as in Dryopteris . It is of a 
darker duller green ; its stalk is more scaly at the 
lower part—green instead of purple, and the clusters 
of sori more densely crowded. It is found commonly 
in Derbyshire, near Matlock, and abundantly in Cum¬ 
berland, Westmoreland, Yorkshire, and Lancashire. 
It does not seem to have been found in Scotland or 
Ireland. 
The mealy dust which characterises this fern is a 
beautiful object under the microscope, each slender 
stem supporting a globular head ; but as this appear¬ 
ance soon goes off when the specimen dries, it is best 
to examine freshly-gathered plants. 
There seems to be no great difficulty in cultivating 
this fern in the ordinary soil of gardens, although it 
