I'm pretty familiar with Didymodon sinuosus now, and probably see it in the majority of tetrads I visit in SE Glamorgan, but have recently come across a couple of odd examples of what I think can only be this species.
The first was on limestone rubble in secondary woodland near Wenvoe garden centre on 22nd January. The plants were dry and shrivelled so I took a sample home to confirm the ID, and on moistening it became obvious that a couple of the older leaves had clusters of filamentous gemmae along the costa. I'm afraid the photos are not the best, see below.
I can't find any mention of gemmae in this species, except for a fairly recent reference to protonemal gemmae in Rare and Interesting No. 11. This reference also mentions that "Correns (1899) had reported gemmae from a Sussex plant in the late 1880s", but doesn't say whether these were protonemal or not. Has anyone else seen similar plants?
The following day, I collected a very shiny-leaved sample from Llanilid Churchyard. I didn't even suspect this was D. sinuosus, which is usually much more opaque-looking, but having checked it microscopically I think it must be. The leaf cells are not at all papillose, even near the leaf tips, but otherwise they look spot on for D. sinuosus with their wavy, notched margins. Smith mentions that the cells are papillose in this species, which fits with the opaque plants I'm faimilar with. I guess this is just natural variation; many of the Didymodon species seem to vary in the degree of 'papillosity' of their cells.
Any comments welcome, thanks.