The capsules look rather wide for S apocarpum and I'd expect at least the bracts to have hairpoints. I suspect it's S rivulare. I'll have a think about whether there are any other characters.
Spore size may help and you've got capsules. If they are 15 microns or less it rules out rivulare. If they are 16 or greater, at least that points to rivulare. I see more rivulare than apacarpum in those sorts of habitats , especially where submergence is frequent. S. apocarpum looks neater in appearance to me.
I'm not very familiar with variation in either species so thanks for the help - I'll look at some spores and let you know, hopefully they'll be developed enough
Spores mostly 12-13 microns
Sounds like apocarpum. What do you think Sam? If it is, it's a useful lesson for us.
Capsules still not mature which might explain spore size?
Unripe spores don't count!
Cells in basal quarter of leaf should be non sinuose in rivulare and sinuose in apocarpum. Cross section of costa just up from leaf base should have differentiated larger adaxial cells in rivulare and should be uniform in apocarpum. Exothecial cells thick walled and with trigones in rivulare and thinner walled in apocarpum. Lots of characters but all subtle!
Thanks Sam I check these characters out...
Neither the exothecial cells nor the basal cells in the leaf are terribly convincing for either case to me, but the differentiated adaxial cells of the costa fit your description for rivulare perfectly. I'll post a few pics later.
I agree - it looks like S rivulare on costa features. The capsule widens considerably on dehiscence, so its current length is not really an indicator of S apocarpum.
Thanks for the help sorting this one out. Not rare I know, but hopefully it will have been as instructive for others as it was for me.