compressus

Status Flowers UK Biodiversity Action Plan Priority Species Nationally Scarce. IUCN Threat category: Endangered (2005).

Taxonomy Magnoliopsida: Scientific name: L. Common names: Grass-wrack Pondweed, Dyfrllys Camlaswellt

Potamogeton is a genus with 21 species described for the British Isles. The species are often confused, but Leaves although there are 25 hybrids recorded hybridisation 3-6mm is uncommon and most hybrids are rare. The most wide with detailed account in recent years is that of Preston 5 main (1995) which is recommended for full differentiation veins Stipules from other species. See also Stace (1997) and the more ecological accounts in Preston & Croft (1997) and Stewart et al. (1994). Stems compressed Biology & Distribution and narrowly Potamogeton compressus is a native, aquatic, winged rhizomatous perennial and has been recorded from a wide range of aquatic habitats. At present it occurs in the Welsh borders, scattered sites in central England, coastal areas of Norfolk, and is known from one site in Scotland, but it is widespread elsewhere in Europe. Figure 1. Potamogeton compressus (from J. E. Smith & J. Sowerby (1902). English Botany. 3rd ed. London) It occurs in lowland water-courses that are generally still or slowly flowing, mesotrophic and slightly to Identification & Field survey moderately base-rich. Thus, it can be present in lakes, For accurate identification of Potamogeton, it is sluggish rivers, ditches, canals and flooded mineral necessary, first of all, to examine the leaf morphology, workings. including the range of leaf-shape, the leaf-blade venation, and the morphology of the basal sheath / Some of the most vigorous surviving populations are stipules. The middle and lower submerged leaves are in canals, especially the Montgomery branch of the the diagnostic ones. ̀Veinś refers to the midrib and Shropshire Union Canal. However, it appears to have lateral veins, which run ± parallel to it for nearly the been in gradual decline over the last hundred and whole leaf length. Fruit lengths include the beak. fifty years, being almost extinct in lakes and rivers and decreasing in the ditches of grazing marshes. Fresh material is best for determination, the stipule characters being difficult to see in dried material, but The decline has been caused by several factors are sometimes necessary for a determination. including eutrophication (nutrient enrichment) of its aquatic habitats, recreational pressures such as Potamogeton compressus is a totally submerged aquatic boating and the resulting pollution and churning up that has very flattened, branching stems that can of sediment, and the neglect of ditches and canals. reach up to 90 cm in length. The leaves are long and slender; they usually have five main veins running Local action plans along their length. The small flowers and fruits are UK Biodiversity Species Action Plan (October 2002) produced sparingly, with in the ditches of http://www.ukbap.org.uk. grazing marshes perhaps producing more than those in other habitats. Flowers are best seen in July, and References fruit, which are useful for identification, in the first Preston, C. D. (1995). Pondweeds of Great Britain and half of August. New plants are generally formed from Ireland. Botanical Society of the British Isles handbook turions (reduced branches) which begin to develop no. 8. Botanical Society of the British Isles, in late June and are the means by which they over- London. winter. Preston, C. D. & Croft, J. M. (1997). Aquatic Plants in Great Britain and Ireland. Harley Books, Colchester. Key characters Preston, C. D., Pearman, D. A. & Dines, T. D., eds. Rhizomes absent. Stems to 90 cm, strongly (2002). New Atlas of the British & Irish flora. Oxford compressed and sometimes narrowly winged at University Press, Oxford. the edges, slightly swollen just below the nodes. Stace, C. A. (1997). New Flora of the British Isles. 2nd Leaves all submerged, linear, 3-6 mm wide, 3.5-13.5 edition. Cambridge University Press, Cambridge. cm long, acute to rounded at apex, with 5 main Stewart, A., Pearman, D. A. & Preston, C. D., eds. (1994). veins (of which 2 are submarginal), and many fine Scarce plants in Britain. Joint Nature Conservation but conspicuous strands between them. Even on Committee, Peterborough. the same stems, the apex of the lower leaves can be truncate and the upper mucronate. Stipules 20-55 Additional photographs are available on the ARKive mm, open, translucent, but opaque and buff-coloured website; http://www.arkive.org/species when dry, and although persistent, soon eroding to fibrous strands at the apex, and finally splitting into a V-shape remnant. Turions 25-45 x 3.5-8 mm, terminal at the ends of short axillary branches. Inflorescences 11-25 x 2.5-4.5 mm, peduncles 28-95 mm, slender or robust, compressed. Fruit 3.4-4 x 2.1-3.0 mm with beak 0.4-0.7 mm, recurved.

Differentiation from similar species The flattened stems and the numerous sclerenchymatous strands in the lamina of the leaf, giving the appearance of a many-veined leaf, distinguishes P. compressus from all British species except P. acutifolius. has 3 (not 5) main veins to the leaves, which are usually acuminate not mucronate at their apex.

The mature fruits of P. acutifolius usually have a basal wart or tooth, usually with an erect symmetric beak, whereas P. compressus has unwarted fruit, with an asymmetric, curved beak.