Willows, also called sallows and osiers, of the genus Salix, comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions.

Willows all have abundant watery bark sap, which is heavily charged with salicylic acid, soft, usually pliant, tough wood, slender branches, and large, fibrous, often stoloniferous roots.

Most species are deciduous; semi-evergreen willows with coriaceous leaves are rare, e.g. Salix micans and S. australior in the eastern Mediterranean.

The staminate (male) flowers have neither calyx nor corolla; they consist simply of stamens, varying in number from two to 10, accompanied by a nectariferous gland and inserted on the base of a scale which is itself borne on the rachis of a drooping raceme called a catkin, or ament.

The pistillate (female) flowers are also without calyx or corolla, and consist of a single ovary accompanied by a small, flat nectar gland and inserted on the base of a scale which is likewise borne on the rachis of a catkin.

The seeds are tiny, plentiful, carried by wind and water, and viable only for a few days; they require warm and moist conditions to take root.

The wood was used by some Native American tribes to start fires by friction, the shoots to weave baskets, and both the branches and stems to build various items including fishing weirs.

[citation needed] Interpreting Mesopotamian cuneiform texts is a challenge, especially when looking for something as specific as a species of plant being used to treat a recognisable condition.

One, as part of an elaborate recipe for a poultice to "make the met supple," which involved 36 other ingredients including "fruit of the dompalm, beans and amaa grains."

The second is as part of a treatment for the "Great Debility," when "rush from the green willow tree" is combined with ass's semen, fresh bread, herbs of the field, figs, grapes and wine.

Finally, it is used as a stiffening agent in a concoction of "fat flesh, figs, dates, incense, garlic and sweet beer" to put the heart into proper working order and make it take up nourishment.

[30] The Roman author Aulus Cornelius Celsus only mentions willow once: the leaves, pounded and boiled in vinegar, were to be used as treatment for uterine prolapse, but it is unclear what he considered the therapeutic action to be; it is unlikely to have been pain relief, as he recommended cauterization in the following paragraph.

[31][32] Nicholas Culpeper, in The Complete Herbal,[33] gives many uses for willow, including to staunch wounds, to "stay the heat of lust" in man or woman, and to provoke urine ("if stopped"), but he makes no mention of any supposed analgesic properties.

His recommendation to use the burnt ashes of willow bark, mixed with vinegar, to "take away warts, corns, and superfluous flesh," seems to correspond with modern uses of salicylic acid.

William Turner's account, written about 1597, focuses on the ability of the leaves and bark to "stay the spitting of blood, and all other fluxes of blood", if boiled in wine and drunk, but adds a treatment for fever, saying: "the green boughs with the leaves may very well be brought into chambers and set about the beds of those that be sick of fevers, for they do mightily cool the heat of the air, which thing is a wonderful refreshing to the sick patients.

"[34] In 1763, Reverend Edward Stone, of Chipping Norton, Oxfordshire, England, sent a letter to the Royal Society describing his experiments with powdered bark of white willow (Salix alba).

[37] In his novel The Mysterious Island (1875), the French novelist Jules Verne outlined the state of scientific knowledge concerning medicinal uses of willow when one of his characters, Herbert (Harbert) Brown, was suffering from a fever induced by a bullet wound: "The bark of the willow has, indeed, been justly considered as a succedaneum for Peruvian bark, as has also that of the horse-chestnut tree, the leaf of the holly, the snake-root, etc.

The first lasting evidence that salicylate, from willow and other plant species, might have real medicinal uses came in 1876, when the Scottish physician Thomas MacLagan experimented with salicin as a treatment for acute rheumatism, with considerable success, as he reported in The Lancet.

[42] The discovery of acetanilide, in the 1880s, gave rise to an 'acetylation' craze, where chemists experimented with adding an acetyl group to various aromatic organic compounds.

[43] Back in 1853, chemist Charles Frédéric Gerhardt treated the medicine sodium salicylate with acetyl chloride to produce acetylsalicylic acid for the first time.

[44] More than 40 years later in 1897, Felix Hoffmann created the same acid (in his case derived from the Spiraea plant),[45] which was found in 1899 to have an analgesic effect.

One modern field guide claims that Native Americans across the Americas relied on the willow as a staple of their medical treatments, using the bark to treat ailments such as sore throat and tuberculosis, and further alleging that "Several references mention chewing willow bark as an analgesic for headache and other pain, apparently presaging the development of aspirin in the late 1800s.

[46] In the early 20th century, Maud Grieve described using the bark and the powdered root of white willow (Salix alba) for its tonic, antiperiodic and astringent qualities and recommended its use in treating dyspepsia, worms, chronic diarrhoea and dysentery.

[47] Like other herbalists, she makes no mention of it having any analgesic effect, despite widespread awareness of aspirin by this time, and she considered tannin to be the active constituent.

[51] There are now many papers, books and articles repeating the claim that the ancients used willow for pain relief, and numerous willow-based products can be purchased for this purpose.

[52] Modern research suggests that only the mildest analgesic effect could be derived from the use of willow extract, and even that may be due to flavonoids and polyphenols as much as salicylic acid.

[citation needed] In traditional pictures of Guanyin, she is often shown seated on a rock with a willow branch in a vase of water at her side.

The image is sent to the nether world, where the disembodied spirit is deemed to enter it, and give the desired information to surviving relatives on its return.

A gisaeng (Korean courtesan) named Hongrang, who lived in the middle of the Joseon Dynasty, wrote the poem "By the willow in the rain in the evening", which she gave to her parting lover (Choi Gyeong-chang).