Taxonomy
Zygodon forsteri Knothole Moss or Forster’s yoke-moss. © F Rumsey
This species has long stood out as distinct, being the only autoicous (with male and female organs on the same plant) European species within the genus and also the only one with smooth leaf cells. However, it has only been since the advent of molecular studies that its generic placement has been questioned.
Just over a decade ago Goffinet and Vitt (1998) recognized Codonoblepharon as a genus independent from Zygodon. At that time they speculated that Z. forsteri could have evolved from a Zygodon s.str. ancestor, and explicitly excluded it from Codonoblepharon. Later though, on the basis of new molecular evidence, Goffinet et al. (2004) moved it to Codonoblepharon.
Matcham and O’Shea (2005) excluded this species either from Codonoblepharon or Zygodon on the basis on its white rhizoids, wider cells, negative KOH reaction of the laminal cell walls, and from the former, mainly Southern hemisphere genus, on its temperate Northern Hemisphere distribution. The inference is that it would thus warrant recognition as a distinct, but as yet un-named genus.
Forms with the stout leaf nerve excurrent into a sharp mucro have been described as var. sendtneri but this character is highly variable even on the same shoot and the variety is not generally now recognised.
The rather poorly known Z. madeirensis Dixon & Luisier, described from Madeira, is probably conspecific but requires further study (Hill et al., 2006).