Onagraceae

A crisply defined family, easily recognised once the pattern is known: flower parts in fours, an inferior ovary that reads as a long stalk below the petals, and often a four-lobed stigma. Around 650 species, mostly herbs, distributed worldwide with a centre of diversity in western North America.

The family includes some conspicuous colonisers. Fireweed is the plant that carpets burned ground and bomb sites — it is called rosebay willowherb in Britain, where it spread along railway cuttings and through the ruins of the Blitz, and Ivan chai in Russia, where the fermented leaf has been drunk as tea for centuries.

This family is unusually coherent medically, because one molecule carries almost the whole of it. Oenothein B — a macrocyclic dimeric ellagitannin named for Oenothera — occurs across the family and is the basis of its principal use: benign prostatic hyperplasia and lower urinary tract symptoms. It inhibits 5α-reductase and aromatase, the same enzymatic targets that conventional BPH drugs address, and it is anti-inflammatory in prostate tissue models.

European practice built on this long before the chemistry was known. Epilobium parviflorum and related small-flowered willowherbs are established prostate remedies in German and Austrian herbal medicine, and fireweed carries the same use. The evidence remains largely preclinical, with the mechanism considerably better characterised than the clinical outcome.

The family’s other well-known product is different in kind. Evening primrose seed oil is a source of gamma-linolenic acid, sold for eczema, mastalgia and menopausal symptoms — a fatty-acid story with no relation to oenothein B, and one where the trial evidence has been consistently disappointing for eczema and modest at best elsewhere.

Oenothein B and its relative oenothein A are the family markers, and macrocyclic ellagitannins of this kind are rare enough elsewhere to be diagnostic. They sit within a broad ellagitannin and flavonoid background — myricetin, quercetin and kaempferol glycosides, hyperoside, isoquercitrin, myricitrin, with gallic and ellagic acids released on hydrolysis.

The seed oils are a separate chemistry entirely: gamma-linolenic acid, unusual among plant oils, concentrated in Oenothera seed.