Potamogeton
Description
Aquatic herbs, rarely with terrestrial forms, perennial, forming more or less persis-tent shoot complexes, under seasonal conditions partly annual, reproducing by seeds.
Leaves alternate, but subopposite in the flowering region, margin entire or dentate, only submerged or both submerged and floating leaves present, all leaves with stipules;
Inflorescence a bractless cylindrical spike, long-peduncled, mostly emergent, with 1-20 contiguous or distant whorls of flowers, whorls 1-4-flowered;
Flowers small, yellow, green or brown, actinomorphic.
Fruit a drupaceous achene, sessile, 1-seeded, indehiscent, ovoid to subglobose, with a short terminal beak, surface smooth or ridged or tuberculate;
Distribution
Amazonia present, Asia-Temperate, Asia-Tropical, Cosmopolitan present, Japan present, North America present, West Europe present
Cosmopolitan, highest species densities in North America, West Europe, Siberia and Japan, almost lacking in Amazonia, c. 70 species; in Malesia 13 species, 2 of which may be recently introduced; 1 species is endemic to the region.
Anatomy
Besides morphological characters, stem anatomical characters are most useful for identification of Potamogeton specimens, particularly such ones in fragmentary state on old herbarium sheets. Five groups of characters can be used for species identification (see Raunkiaer 1903; Hagström 1916; Wiegleb 1990): shape and size of the stele, shape of the endodermis cells, numbers and size of interlacunar and subepidermal bundles, shape of the pseudohypodermis.
Taxonomy
All Potamogeton species, living in a variable habitat, show a high degree of phenotypic plasticity, thus the vegetative habit is often extremely variable. This led to the description of a large number of forms, varieties, ‘hybrids’ and ‘species’ without taxonomic value. In general it is difficult to identify sterile specimens of broad-leaved species without checking the stem anatomical characters. In narrow-leaved species specimens without fruits or turions often cannot be identified to the species level. Despite the high diversity of growth forms subdivision of the genus is complicated by the irregular distribution of supposed key characters. Wallman (1812) was the first to recognise the special position of P. crispus. A subdivision of the genus based on the shape and size of leaves and stipules was proposed by Koch (1837). Gay (1854) excluded the section Enantiophylla as a separate genus Groenlandia. Raunkiaer (1896) divided remaining Potamogeton into two subgenera, namely Coleogeton and Potamogeton (‘Eupotamogeton’). The difference is as follows:
Subgenus Coleogeton — Submerged leaf phyllodial. Stipules always adnate to the leaf for largest part. Winter buds tuber-like on horizontal shoots, the renewal shoot and the parent shoot partly adnate. Vascular bundles of the peduncle each with an individual endodermis; cortical bundles absent (in Malesian species); hypodermis often absent. Peduncle not erect; pollination at water surface. Pollen grain size and size of the lumina larger.
Subgenus Potamogeton — Submerged leaf phyllodial or laminar. Stipules mostly axillary, forming an open or fused ochrea, rarely adnate in the lowermost part. Winter buds as turions in the axils of vertical shoots, or as turions, tubers or complex structures on horizontal shoots. Vascular bundles of the peduncle without endodermis, each with a fibrous sheath; hypodermis present, even if absent in vegetative stem. Peduncle erect; pollination above water surface.
Coleogeton has also been treated as a separate genus Stuckenia by Börner (1912) respectively Coleogeton by Les & Haynes (1996).
Hagström (1916) presented a sophisticated classification into sections and subsections, which was a further development of Graebner’s treatment (1907). It was based both on morphological and stem anatomical characters. Data on endocarp structure (Aalto 1970), chromosome number (Les 1983), pollen type (Sorsa 1988), flavonoid chemistry (Haynes 1985; Les & Sheridan 1990a) as well as a re-evaluation of stem anatomy (Wiegleb 1990), morphology and life history partly corroborate, partly question the grouping of Hagström. Therefore, Wiegleb (1988) proposed to use an informal grouping into species groups. Further attempts to elucidate the infra-generic relations were made by Les & Sheridan (1990b) and Hettiarachchi & Triest (1991). All results mentioned are likewise speculative and based on insufficient evidence. At present no acceptable classification into sections and subsections is available.
In the following a conservative approach to species delimitation is adopted. There are several specimens that do not fit exactly into the species as circumscribed here. These specimens are mentioned in the notes of the most similar species. Supposed hybrids are not included into the species list and key but are described in the notes. The present treatment should be regarded as a working hypothesis and a basis for further study. In Malesia the following species occur:
Subgenus Coleogeton — Submerged leaf phyllodial. Stipules always adnate to the leaf for largest part. Winter buds tuber-like on horizontal shoots, the renewal shoot and the parent shoot partly adnate. Vascular bundles of the peduncle each with an individual endodermis; cortical bundles absent (in Malesian species); hypodermis often absent. Peduncle not erect; pollination at water surface. Pollen grain size and size of the lumina larger.
Subgenus Potamogeton — Submerged leaf phyllodial or laminar. Stipules mostly axillary, forming an open or fused ochrea, rarely adnate in the lowermost part. Winter buds as turions in the axils of vertical shoots, or as turions, tubers or complex structures on horizontal shoots. Vascular bundles of the peduncle without endodermis, each with a fibrous sheath; hypodermis present, even if absent in vegetative stem. Peduncle erect; pollination above water surface.
Coleogeton has also been treated as a separate genus Stuckenia by Börner (1912) respectively Coleogeton by Les & Haynes (1996).
Hagström (1916) presented a sophisticated classification into sections and subsections, which was a further development of Graebner’s treatment (1907). It was based both on morphological and stem anatomical characters. Data on endocarp structure (Aalto 1970), chromosome number (Les 1983), pollen type (Sorsa 1988), flavonoid chemistry (Haynes 1985; Les & Sheridan 1990a) as well as a re-evaluation of stem anatomy (Wiegleb 1990), morphology and life history partly corroborate, partly question the grouping of Hagström. Therefore, Wiegleb (1988) proposed to use an informal grouping into species groups. Further attempts to elucidate the infra-generic relations were made by Les & Sheridan (1990b) and Hettiarachchi & Triest (1991). All results mentioned are likewise speculative and based on insufficient evidence. At present no acceptable classification into sections and subsections is available.
In the following a conservative approach to species delimitation is adopted. There are several specimens that do not fit exactly into the species as circumscribed here. These specimens are mentioned in the notes of the most similar species. Supposed hybrids are not included into the species list and key but are described in the notes. The present treatment should be regarded as a working hypothesis and a basis for further study. In Malesia the following species occur:
- Subgenus Coleogeton (Rchb.) Raunk.: P. pectinatus
- Subgenus Potamogeton: P. maackianus, P. crispus, P. furcatus, P. oxyphyllus, P. pusillus, P. octandrus, P. solomonensis, P. nodosus, P. distinctus, P. papuanicus, P. lucens, P. wrightii, P. perfoliatus
Cytology
The chromosome numbers mentioned under several species are all based on counts of non-Malesian material. Only the most reliable counts are reported (Hatusima 1961; Preston 1995). Chromosome counts are often obscured by methodological difficulties like disregarding the B-chromosomes (Preston 1995) and false determination (Wiegleb 1988). A lot of deviating numbers have been reported for most species (see Les 1983; Preston 1995).