Oenanthe fistulosa

Tubular water-dropwort is a hairless, stoloniferous perennial growing up to 80 cm tall with brittle, hollow, inflated cylindrical stems 0.5 cm in diameter, which are constricted at the nodes (hence the specific name fistulosa). Unlike some other water-dropworts, it has no swollen tubers among the roots, but it can reproduce vegetatively by the lengthy stolons.^[1]

The leaves vary widely in shape, with the upper ones being typically pinnate and having narrow, almost linear segments, while the lower ones can be 2- or even 3-times pinnate, with broader, flat leaflets, more like those of other umbellifers. The leaf stalks of the upper leaves are fistular, like the stem, and longer than the pinnate leaf blade.^[2]

It flowers between July and September in northern Europe, with clusters of 2-4 umbels about 1 cm in diameter, each of which has numerous white to pinkish flowers. A distinguishing feature is that, unlike many other umbellifers, it has no bracts but only small bracteoles on the individual umbels. The rays (stalks to the umbels) are 10–30 mm long and become thicker after flowering.^[2][1]

Plants are monoecious, with hermaphroditic and male flowers on the terminal umbels, and only male flowers on the lateral umbels. Each flower has 5 unequal petals with the larger, outer ones radiating, 5 stamens and 2 prominent styles arising from a swollen base (stylopodium) at the top of the ovary. After flowering, the flower stalks and fruits expand to form distinctive pink balls with the remains of the styles projecting in pairs from the surfaces of the cone-shaped fruits.^[2][1]

The scientific name for tubular water-dropwort was coined by Peter Artedi, who developed the modern binomial classification system, and defined the species on the basis of its involucral characters.^[3] It was published after Artedi's death by Linnaeus in Species Plantarum, in 1753, and that name still stands, although numerous others (synonyms) have been proposed over the years, including Phellandrium fistulosum (L.) Clairv. (1811), O. filipendula Dumort. (1827) and Selinum fistulosum (L.) E.H.L. Krause (1904). A full list can be found in the Synonymic Checklists of the Plants of the World.^[4]

A few subspecies and variants have been named, such as forma submersa (Glück, 1936), subspecies fistulosa (Linnaeus, 1753) and var. tabernaemontani ((C.C. Gmel.) DC., 1830), but none is in common use now.

There are no reported hybrids.^[5]

Its chromosome number is 2n = 22.^[2]

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. "Fistula" is the Latin for tube, and refers to the shape of the stem. The "dropwort" part of the common name is a reference to the tubers produced amongst the roots of certain other species in the genus.^[6]

It can be distinguished from other British species of Oenanthe by its inflated hollow stems (those of lachenalii and pimpinelloides are solid); the absence of bracts (present in crocata, lachenalii and pimpinelloides); the fruit being globose with all fruits sessile (not so with crocata, silaifolia, pimpinelloides or lachenalii); and the fruits being less than 4 mm long (more in crocata and fluviatilis).^[7]

The global range of tubular water-dropwort is from Europe through the northern half of Africa to western parts of Asia. It occurs no further north than southern Scandinavia and is rare as an introduction beyond its natural range. In Britain and Ireland it is found mainly in the lowland eastern counties, becoming increasingly rare towards the uplands of the north and west.^[2][8] In France the pattern is similar, with the main populations in the southern lowlands, becoming rarer towards the upland regions around the Alps.^[9]

The IUCN threat status is Least Concern (LC) (as assessed in 2013), both globally and in Europe.^[10][11] In France, it is similarly considered unthreatened nationally, but in some reasons it is declining, notably in Limousin, where it is Critically Rare (CR), and Midi-Pyrénées, where it is Endangered (EN).^[9] It is also declining in some other European countries, such as Slovenia, where it is classed as EN.^[12]

In Britain it is classified as Vulnerable (VU), while in England it is believed to have declined by 35% in its area of occupancy between the 1960s and the 1990s. This decrease appears to have continued in some English counties, such as Kent, where the number of sites has shrunk by 60% between the 1970s and 2005, despite it being a Biodiversity Action Plan species.^[7]