Phylum: Charophyta   Family: Closteriaceae

Closterium angustatum var. sculptum (Raciborski) Ruzicka 1977

Differs from the nominate in that the striae (costae) have the crests interrupted with thickenings giving a corrugated or knotted appearance. Brook & Williamson (2010, p. 204, pl. 85: 5-6 & pl. 87: 1) comment: 'Found only in acid environments similar to those inhabited by the nominal vatiety' i.e. quite acidic. The current finds from Lewis were in slightly acidic waters. It would appear to be very scarce with few recent finds.

References: 
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.
Ruzicka, J. (1977) Die Desmidiaceen Mitteleuropas, Volume 1, Part 1.