Taxonomy of this group was elucidated by Frisvoll
(1988) and Blockeel (1991). Only R. heterostichum s. str. has been
recorded in Cornwall, despite routinely checking leaf-sections
and other characters of most plants recorded since 1993 on the
assumption that R. affine (Schleich. ex
F.Weber & D.Mohr) Lindb.(which is known in Devon) and
perhaps R. sudeticum (Funck)
Bruch & Schimp. 'should' also occur. Nevertheless,
although Porley & Hodgetts (2005: 409) appear to imply
that R. affine occurs on
Bodmin Moor it has not been recorded in Cornwall. Records from
before the revision by Blockeel (1991) are treated here as the
R. heterostichum s. l. (unless old
specimens were revised), along with more recent records which
have not been carefully checked for the other segregate
species.
Some Cornish plants show the characters of R. heterostichum fo. obtusum, which was
regarded as a distinct species by Frisvoll. Large plants of R. heterostichum from an
old flooded quarry on Carn Marth, with a broad nerve and blunt
leaves, were superficially similar to R. aquaticum, but their
leaf-cells lacked all trace of papillae.
First vice county records of the s. l. species (see below):
*1: Mean Rock, N. of Constantine, 1837, EAW (TRU) (Paton 1969a:
730).
*2: Helman Tor, 1891, RVT (B) (Paton 1969a:
730).