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A DESMID FORM NEW TO THE BRITISH FLORA
Cosmarium tetrachondrum Lundell, unnamed forma
This has been re-written with additional information for these web pages.
In March 1993 I received a small sample from Mr. Alan Joyce collected from a bog in north-west Sutherland, Scotland. It contained a diverse collection of microscopic algae typical of acidic bog habitats, including an interesting population of the Class Zygnemaphyceae (formerly Conjugatophyceae) which includes the desmids. The desmid families have some 35 genera many of which were present in the sample, the genus Cosmarium being abundant. Cosmarium is one of the largest genera about 230 species, varieties and forma have been recorded from the British Isles and more than 1,250 world-wide.
Half the sample was preserved, the remainder decanted into a small petri dish and examined the day of arrival. It was then kept on the sill of a north-facing window and examined over a period of several weeks, this method will often allow cell division to be observed, and occasionally conjugation will be seen.
Every slide of material examined had a few Cosmarium ornatum Rails 1848, a number of -
C. quadratum Ralfs 1848, and also of C. tetrachondrum Lund.forma the subject of this paper. The latter algal form has been presented by several authors:- Gronblad ( 1921); Huzel ( 1936); Prescott et al. (1981 figure only); & Coesel (1991). But never formally described.
In the Netherlands it was encountered in some slightly acid, oligo-mesotrophic moorland pools, but only in the first half of this century, P.Coesel (personal communication). R.Lenzenweger informed me it has not been recorded from Austria or Switzerland.
The semi-cells differ from the type in having rounded undulating margins with four very, small granules within each margin and with a truncate or slightly convex apex, the walls are covered with pores. As with the type there are two prominent granules near the isthmus just above the deep closed sinus, and a single pyrenoid in the centre of each semi-cell. The photomicrographs are in the picture gallery Cells 20-23 µm long; 24-26 µm broad; 8-10 µm thick;
isthmus 7-7.5 um broad.
This is the first recording of this form from a British habitat, it is not in A Check-List of Desmids of the British Isles (1991 ).
I have written to Carlos E de M Bicudo the surviving author of A Synopsis of North American Desmids part ll section 3. to see if he knows anything of the geogaphical distribution of the forma. If any reader of this page can help I will be much obliged.
Acknowledgements
The author is grateful to Mr. Alan Joyce for the samples from Scotland. To Dr. P.F.M. Coesel of the University of Amsterdam for his helpful comments, to Prof. R.Lenzenweger for information on distribution, and to Dr. J.W.G. Lund for figures from The Fritsch Collection of Algal Illustrations (Freshwater Biological Association, England).
References
Brook A.J. and Williamson D.B. ,4 Check-List of Desmids of the British Isles (1991) Freshwater Biological Association.
Coesel P.F.M. (1991) De Desmidium van Nederland. 4. (88pp). KNNV, Utrecht.
Gronblad R. (1921 ) Acta Soc. Fauna Flora. Fenn.49.
Huzel C. (1936) Beltrag zur Kenntnis der Mikrokopischen Pflanzenweit der Rauhen Wiese bei Böhmenkirchh. Veröffenlichungen der Wurttemberg Landesselle f'ür Naturscutz. Heft 13 (pp 5-117).
Prescott G.W. Croasdale H.T. Vinyard W.C. & Bicudo C.E. De M. (1981 ) A Synopsis of North American Desmids Part II Section 3. University of Nebraska Press.
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