Potamogeton berchtoldii

Potamogeton berchtoldii, common name small pondweed is an aquatic plant.
Description.
Small pondweed is a fine-leaved pondweeds with a bushy habit. It lacks a perennial rhizome and the plants die back in winter into a large number of resting buds known as turions. The stems are very slender and do not usually exceed 60 cm. The leaves are flat, usually 25–50 mm long, and less than 2 mm wide, usually with a broad band of lacunae along each side of the midrib and coloured pale green, olive green or brownish green. The open stipules and shining nodal glands can be important characters in identification.
Small pondweed is similar to several other "Potamogeton" species, especially "P. pusillus", and use of a good key such as Preston (1995) is strongly recommended. It could also be confused with "P. trichoides" and "P. obtusifolius".
Taxonomy.
Small pondweed was first named by the German botanist Franz Xaver Fieber in 1838. It is named after the 19th century Czech botanist Friedrich von Berchtold.
This is one of a group of rather similar fine-leaved species that also includes "P. pusillus", "P. hillii" and "P. ochreatus". Some authors consider it a form of "P. pusillus". 
Subspecies include:
Hybrids have been described with "P. trichoides" ("P". × "franconicus" ), "P. pusillus" ("P". × "mucronulatus" ), "P. natans", "P. polygonifolius", "P. coloratus" ("P". × "lanceolatus" ), "P. perfoliatus" ("P". × "mysticus" ), "P. natans" ("P". × "variifolius" ), and "P. acutifolius" ("P." × "sudermanicus" ), but all are rare.
Distribution.
"Potamogeton berchtoldii" is native to Europe (including Britain, Ireland, continental Europe, Scandinavia), the Middle East, North America (Canada, USA), Asia (Bhutan, China, Japan, Kazakhstan, Korea, Tajikistan, Turkmenistan, Russia, Uzbekistan), Asia Minor and the Middle East (Iraq, Iran, Turkey).
Ecology and conservation.
"Potamogeton berchtoldii" has a wide ecological tolerance, growing in lakes, ponds, ditches, slow-flowing streams and rivers, temporary pools and sometimes appearing in artificial habitats such as cattle troughs. It is a good colonist and may quickly exploit new habitat. The production of large numbers of both turions and seed likely allows it to quickly exploit new or disturbed habitat, but in stable environments it is likely to be outcompeted by more robust plants.
In Britain, small pondweed is common and widespread. It is probably under-recorded due to its relatively small size and confusion with "P. pusillus".
Although it does not occur in very acid environments, small pondweed is otherwise tolerant of a wide range of water chemistry and seems able to survive in eutrophic conditions. However, it is less tolerant of high nutrient levels than the related and very similar "P. pusillus". It is the only British fine-leaved pondweed likely to be encountered in oligotrophic lakes, where it generally grows in deeper water with "Isoetes" and "Nitella" spp.
Cultivation.
Small pondweed is not in cultivation, though it is likely to spontaneously colonise new ponds, probably via waterfowl. It is unlikely to persist for long in this environment, however, unless competing plants are regularly cleared. Its small stature and noninvasive habit make it suitable for growing in a pond or aquarium.