Phylum: Basidiomycota Family: Urocystidaceae
Urocystis ranunculi (Libert) Moesz, 1950
Sori in leaves and stems of creeping buttercup (Ranunculus repens), which burst revealing a black spore-mass. Probably fairly common in the Outer Hebrides but under-recorded.
References:
Ellis, M.B. & Ellis, J.P (1997 new enlarged edition) Microfungi on Land Plants: An Identification Handbook.