Ruppia
Description
Annual or perennial submerged aquatic herbs.
Leaves linear, distichous, with very many tannin cells;
Inflorescence terminal, consisting of a peduncle which has at its top a two-flowered spike.
Flowers placed at opposite sides of the axis, but very closely together, bisexual, without a perianth, consisting of 2 opposite stamens and 4-~carpels.
Stamens consisting of one (sub)sessile, bilocular anther;
Ovule solitary, pendulous, campylotropous.
Fruit an achene, sessile or stalked (podogyne and fruit form a morphological entity without abscission zone), symmetric to very asymmetric;
Distribution
Asia-Tropical, Widely distributed in temperate and tropical regions all over the world present
Widely distributed in temperate and tropical regions all over the worldMalesia
Taxonomy
Circa 10 species are known. As a consequence of great morphological variation between populations, partly due to environmental differences and partly genetically determined, the taxonomy of the genus is still unsatisfactory. Another difficulty is that in the past it was not recognised that in the herbarium material the flowering and fruiting organs were not always in the same stage, so the number of described varieties is large; most of them cannot be maintained. However, many investigators have concluded from the chaotic taxonomic situation that the best solution of the problem was to consider the genus as consisting of one very variable species; later investigations, based on the study of live plants, herbarium material and chromosome analyses in Europe , Australia and New Zealand have shown that this is not correct. So far there are no special taxonomic studies of any tropical population. From East Asia no morphological studies have been published, and the only study that records chromosome numbers gives no indication of the morphological characteristics of the material studied .
The genus Ruppia has been classified in the past in various manners; several authors considered it as a family on its own, the Ruppiaceae, but it has also been regarded as a subfamily of the Potamogetonaceae. According to Jacobs & Brock, l.c., the differences with Potamogeton are not sufficient to warrant a separate position within the Potamogetonaceae sensu stricto. However, a subfamily status is certainly justified, and the status of a family in its own right needs to be considered, particularly as Les, Cleland & Waycott () have found that molecular rbcL data indicate that Ruppia is phylogenetically much closer to Posidonia than to Potamogeton.
From the above follows very obviously that the genus is urgently in need of revision on a world scale. This revision should not only be based on herbarium material, but also on the study of live material cultured under various ecological circumstances; further chromosome and isoenzym studies should be included.
The genus Ruppia has been classified in the past in various manners; several authors considered it as a family on its own, the Ruppiaceae, but it has also been regarded as a subfamily of the Potamogetonaceae. According to Jacobs & Brock, l.c., the differences with Potamogeton are not sufficient to warrant a separate position within the Potamogetonaceae sensu stricto. However, a subfamily status is certainly justified, and the status of a family in its own right needs to be considered, particularly as Les, Cleland & Waycott () have found that molecular rbcL data indicate that Ruppia is phylogenetically much closer to Posidonia than to Potamogeton.
From the above follows very obviously that the genus is urgently in need of revision on a world scale. This revision should not only be based on herbarium material, but also on the study of live material cultured under various ecological circumstances; further chromosome and isoenzym studies should be included.