Leaf of Cyclodictyon laetevirens Photo by Sean Edwards |
The
costa or nerve is one of the most fascinating structures of moss gametophytes.
The structure may be single (e.g. Plagiomniun,
Orthotrichum),
highly expanded (e.g. Paraleucobryum),
double (e.g. Cyclodictyon), very
prominent with dorsal lamellae (e.g. Polytrichum) or
completely absent. The taxonomic value of the costa is widely recognized among
bryologists, for example the presence of a double costa is one of the defining
features of the family Pilotrichaceae.
Among
the members of Pilotrichaceae,
Cyclodictyon is one of the most
charismatic. I came across C. albicans during my undergraduate studies because
it grows in close association with the hornwort Nothoceros vincentianus, near streams in Panamanian cloud forests.
The shortly-acuminate bordered leaves and the double costa are a dead give-away
for the species. In
Europe there is another Cyclodictyon
species, C. laetevirens, commonly named the bright-green Cavemoss. The species is particularly noteworthy in
England due to its rarity. Forming glossy, dark green patches, C. laetevirens is restricted to a few maritime
caves in the Cornish region, where the temperature is unlikely to
ever fall below freezing.
Cyclodictyon laetevirens. Photo by Sean Edwards |
Ron
Porley’s book on rare English bryophytes contains a candid anecdote on the
discovery of C. laetevirens in
England. The species was first collected in 1840 by the botanist J. Ralfs.
Unaware of Ralfs’ collection, W. Borrer also collected the plant in one of the
Cornish caves. He realized this was the first record of the species for England
and when he returned home he asked a local cleric to collect more of the
species to distribute to colleagues. The
zealous cleric wanted to ensure that nobody but his friend collected the
precious plant – and so he deliberately wiped out the entire population! Borrer
became preoccupied about the fate of the plant, probably with some feelings of guilt,
and asked other colleagues if they had come across the species. He discovered
that Ralfs had collected it in a different sea cave, and the two men established
a life-long friendship. The species appears to be restricted to very few sea caves in England,
with a 1998 survey reporting it to cover a total area of 30 ×70 cm on Ralfs’ original cave, growing with Conocephalum conicum (Porley 2013). The species is also found in very few localities in Jura, Scotland.
A major concern for this endangered
species is the vulnerability of its habitat, with sparse protection to Cornish
caves and potential sea-level rises due to global warming. The cave-dweller is,
however, kept ex-situ at the
bryophyte threatened facility at RBGK.
Thanks to R. Porley for his
comments and help in obtaining pictures of C.
laetevirens from S. Edwards.
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