Constant species

Montia fontana, Ranunculus flammula, R. omiophyllus, Sphagnum auriculatum.

Physiognomy

Ranunculus omiophyllus-Montia fontana rills typically have a rather crowded, though not always continuous, cover of vascular plants and bryophytes, much of the growth often submerged in the shallow waters, with a floating or shortly-emergent canopy. Ranunculus omiophyllus is often abundant, its delicate white summer flowers set off against the dark green of the floating leaves, and there is very frequently some Montia fontana. Then, the bronze-coloured floating leaves of Potamogeton polygonifolius are commonly prominent and there can be local patches of Agrostis stolonifera, Glyceria fluitans, Juncus bulbosus, J. articulatus and Callitriche stagnalis and scattered plants of Ranunculus flammula, Myosotis secunda and Stellaria alsine with, more occasionally, Ranunculus repens, Equisetum palustre, Hydrocotyle vulgaris, Galium palustre and Lotus uliginosus. Juncus bufonius and Scirpus setaceus can also sometimes be seen on open patches of wet mud. Bryophytes quite commonly make a substantial contribution to the cover, though the frequent species are very few. Often, there are red-brown clumps of Sphagnum auriculatum growing semi-submerged and freshgreen patches of Philonotis fontana but, apart from occasional Polytrichum commune, other species are sparse with just scattered records for such plants as Calliergon cuspidatum, C. stramineum, Drepanocladus exannulatus, D. vernicosus, Scapania irrigua and Rhytidiadelphus squarrosus.

Habitat

This community is typical of spring-heads and rills at moderate altitudes in south-western Britain, where there is irrigation with circumneutral and probably quite oligotrophic waters.

Phytogeographically, the Ranunculus-Montia com munity can be seen as an oceanic replacement for the Philonoto-Saxifragetum. It has been recorded only from south-western England, Wales and from around the Lake District, though further sampling may well reveal that it occurs throughout the British range of R. omiophyllus, an Oceanic West European plant which is found in central southern and south-west England, through much of north-west England and south-west Scotland. All known stands of the community fall within that part of the country with mild winters, where February minima are by and large more than 1 °C above freezing (Climatological Atlas 1952) though, apart from the presence of R. omiophyllus and the generally Atlantic S. auriculatum such conditions make themselves felt here mostly in a negative way, with the very obvious exclusion of species characteristic of montane springs, such as Saxifraga stellaris, Epilobium anagallidifolium, E. alsinifolium, Pohlia ludwigii, P. wahlenbergii var. glacialis and Bryum weigelii.