Phylum: Charophyta   Family: Closteriaceae

Closterium baillyanum (J. Ralfs) L.A. de Brébisson 1856

First described in Ralfs (1848: 169, pl. 28: 7 c-d) as Closterium didymotocum var. baillyanum and illustrated by Jenner. Shortly after, Brébisson (1856: 151) raised it to species level. A significant feature is the punctation of the entire cell-wall, more pronounced at the apices. Until recently, two of the images displayed here were categorised as var. alpinum. However, comparing the size ranges given by Brook & Williamson (2010: 207), there is considerable overlap. I quote below these dimensions, inclusive of those bracketed:
Var. baillyanum: L: 190-700 µm; B: 20-60 µm; Ap: 14-23 µm; L/B: 8-15.
Var. alpinum: L: 180-580 µm; B: 16-48 µm; Ap: 12-21 µm; L/B: 11-21.
Coesel & Meesters (2023: 46) just replicate these dimensions.
To make sense of these varieties there needs to be a more considered view of their true sizes, showing a distinct separation, otherwise, var. alpinum needs synonymising.
My cell dimensions: L: 316-387 µm; B: 31-33 µm.; Ap: 12.5-14.8 µm; L/B: 10-12.2.
Widespread and common where acidic conditions exit, such as peaty pools, disused peat-cuttings and especially with Sphagnum.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
De Brébisson, L.A. (1856). Liste des Desmidiées, observées en Basse-Normandie.
Ralfs, J. (1848). The British Desmidieae.