Oenanthe silaifolia

Narrow-leaved water-dropwort is a hairless, tuberous perennial growing up to 100 cm tall with a stem that is solid below and hollow in the upper parts, grooved and striated, 0.7 cm in diameter. The upper leaves are once to twice pinnate with linear or lanceolate (sword-shaped) leaflets up to 3 cm long; the lower ones are 2-4 pinnate, with similarly narrow leaflets. The leaf stalks of the lower leaves are long, but the upper leaves have stalks that are shorter than the leaf blade. They do not form a sheath around the stem at their base.^[1][2]

It flowers in June in northern Europe, with primary umbels of 4-8 smaller rounded umbellules about 2 cm in diameter, each of which has numerous white flowers. There are no bracts on the main umbel and many (10-17) small, lanceolate bracteoles at the base of each of the individual secondary umblets. Plants are monoecious, with hermaphroditic and male flowers on the terminal umbels and only male flowers on the lateral ones.^[3] Each flower has 5 unequal petals with the outer ones slightly larger, 5 stamens and 2 prominent styles arising from a swollen base (stylopodium) at the top of the ovary. After flowering, the flower stalks thicken, and the umbels do not become flat-topped in fruit. The fruit are 3-3.5 mm long, cylindrical, with prominent ridges.^[1][4]

The first description of narrow-leaved water-dropwort was by the German botanist Friedrich August Marschall von Bieberstein in his Flora Taurico-Caucasica^[5] in 1819. It has dozens of synonyms (i.e. other authors have subsequently named the same plant, but Bieberstein's name has precedence), including O. biebersteinii Simon (1903), O. peucedanifolia Heuff. ex Boiss. (1873) and Phellandrium lobelii Bubani (1899). A full list can be found in the Synonymic Checklists of the Plants of the World.^[6] A few forms and varieties have also been named, but none is currently accepted. It is not known to hybridise with any other species.^[4]

Its chromosome number is 2n = 22 (based on British specimens).^[1]

The generic name Oenanthe, which comes from the Ancient Greek οίνος, "wine" and άνθος, "flower", was used in ancient times for certain Mediterranean plants and later adopted to describe this genus. The specific epithet "silaifolia" means "with leaves like silaum" and refers to its uniformly narrow leaflets. A "dropwort" is a plant with drop-shaped tubers.^[7][8]

Great care must be taken to distinguish this species from several very similar ones. In Britain, it can most easily be confused with corky-fruited water-dropwort and parsley water-dropwort. Both of these are likely to have slightly broader segments on the lower leaves and will usually have bracts on the main umbels. Unlike the other two, parsley water-dropwort has rays and pedicels that do not thicken in fruit, while corky-fruited water-dropwort is the only one to have umbels that become flat-topped in fruit.^[1] Fine-leaved water-dropwort also has stems which remain hollow as they age, with thin walls (<0.5 mm thick) while the other species often develop solid stems or thicker stem walls.^[3]

The global range of narrow-leaved water-dropwort is centred on Europe, the main populations being in France, England, Spain, Italy and Greece. It extends as far north as The Netherlands, east to the Caspian Sea, and southwards as far as Israel and North Africa. It is not recorded as an introduction beyond its natural range.^[9]

In Britain, it is found from southern England as far north as Yorkshire, and as far west as SE Wales.

In Britain and France, it is classified as Least Concern, meaning that it is not rare nor declining at a particularly high rate. In certain counties and regions, however, it is judged to have a higher threat status.^[10][11] Overall, it appears to be increasing slightly in Britain, although there is uncertainty about the veracity of some records.^[12]

It is considered an axiophyte in all the British counties in which it occurs.^[13]