Aesculus hippocastanum grows to 36 metres (118 ft) tall, with a domed crown of stout branches; on old trees the outer branches often pendulous with curled-up tips. The leaves are opposite and palmately compound, with 5–7 leaflets; each leaflet is 13–30 cm long, making the whole leaf up to 60 cm across, with a 7–20 cm petiole. The leaf scars left on twigs after the leaves have fallen have a distinctive horseshoe shape, complete with seven "nails". The flowers are usually white with a small red spot; they are produced in spring in erect panicles 10–30 cm tall with about 20–50 flowers on each panicle. Usually only 1–5 fruit develop on each panicle; the shell is a green, spiky capsule containing one (rarely two or three) nut-like seeds called conkers or horse-chestnuts. Each conker is 2–4 cm diameter, glossy nut-brown with a whitish scar at the base.
The common name "horse-chestnut" (often unhyphenated) is reported as having originated from the erroneous belief that the tree was a kind of chestnut (though in fact only distantly related), together with the observation that the fruit is most likely to be toxic to horses.
Distribution and habitat
Aesculus hippocastanum is native to a small area in the Pindus Mountains mixed forests and Balkan mixed forests of South East Europe. However, it can be found in many parts of Europe as far up as to Gästrikland in Sweden.
It is widely cultivated in streets and parks throughout the temperate world, and has been particularly successful in places like Ireland, the United Kingdom and New Zealand, where they are commonly found in parks, streets and avenues. Cultivation for its spectacular spring flowers is successful in a wide range of temperate climatic conditions provided summers are not too hot, with trees being grown as far north as Edmonton, Alberta, Canada, the Faroe Islands, Reykjavík, Iceland and Harstad, Norway.
In Britain and Ireland, the seeds are used for the popular children's game conkers. During the First World War, there was a campaign to ask for everyone (including children) to collect horse-chestnuts and donate them to the government. The conkers were used as a source of starch for fermentation using the Clostridium acetobutylicum method devised by Chaim Weizmann to produce acetone for use as a solvent for the production of cordite, which was then used in military armaments. Weizmann's process could use any source of starch, but the government chose to ask for conkers to avoid causing starvation by depleting food sources. But conkers were found to be a poor source, and the factory only produced acetone for three months; however, they were collected again in World War II for the same reason.
The seeds, especially those that are young and fresh, are slightly poisonous, containing alkaloid saponins and glucosides. Although not dangerous to touch, they cause sickness when eaten; consumed by horses, they can cause tremors and lack of coordination.
Though the seeds are said to repel spiders there is little evidence to support these claims. The presence of saponin may repel insects but it is not clear whether this is effective on spiders.
Horse-chestnuts have been threatened by the leaf-mining moth Cameraria ohridella, whose larvae feed on horse chestnut leaves. The moth was described from Macedonia where the species was discovered in 1984 but took 18 years to reach Britain.
The flower is the symbol of the city of Kiev, capital of Ukraine. Although the horse-chestnut is sometimes known as the buckeye, this name is generally reserved for the New World members of the Aesculus genus.
In Germany, horse-chestnuts are often found in beer gardens, particularly in Bavaria. Prior to the advent of mechanical refrigeration, brewers would dig cellars for lagering. To further protect the cellars from the summer heat, they would plant chestnut trees, which have spreading, dense canopies but shallow roots which would not intrude on the caverns. The practice of serving beer at these sites evolved into the modern beer garden.
The seed extract standardized to around 20 percent aescin (escin) is used for its venotonic effect, vascular protection, anti-inflammatory and free radical scavenging properties. Primary indication is chronic venous insufficiency. A recent Cochrane Review found the evidence suggests that Horse Chestnut Seed Extract is an efficacious and safe short-term treatment for chronic venous insufficiency, but definitive randomized controlled trials are required to confirm the efficacy.
Aescin reduces fluid leaks to surrounding tissue by reducing both the number and size of membrane pores in the veins.[medical citation needed]
Safety in medical use
Two preparations are considered; whole horsechestnut extract (whole HCE) and purified β-aescin. Historically, whole HCE has been used both for oral and IV routes (as of year 2001). The rate of adverse effects are low, in a large German study, 0.6%, consisting mainly of gastrointestinal symptoms. Dizziness, headache and itching have been reported. One serious safety issue is rare cases of acute anaphylactic reactions, presumably in a context of whole HCE. Purified β-aescin would be expected to have a better safety profile.
Another is the risk of acute renal failure, "when patients, who had undergone cardiac surgery were given high doses of horse chestnut extract i.v. for postoperative oedema. The phenomenon was dose dependent as no alteration in renal function was recorded with 340 μg kg−1, mild renal function impairment developed with 360 μg kg−1 and acute renal failure with 510 μg kg−1". This almost certainly took place in a context of whole HCE.
Three clinical trials were since performed to assess the effects of aescin on renal function. A total of 83 subjects were studied; 18 healthy volunteers given 10 or 20 mg iv. for 6 days, 40 in-patients with normal renal function given 10 mg iv. two times per day (except two children given 0.2 mg/kg), 12 patients with cerebral oedema and normal renal function given a massive iv. dose on the day of surgery (49.2 ± 19.3 mg) and 15.4 ± 9.4 mg daily for the following 10 days and 13 patients with impaired renal function due to glomerulonephritis or pyelonephritis, who were given 20–25 mg iv. daily for 6 days. "In all studies renal function was monitored daily resorting to the usual tests of renal function: BUN, serum creatinine, creatinine clearance, urinalysis. In a selected number of cases paraaminohippurate and labelled EDTA clearance were also measured. No signs of development of renal impairment in the patients with normal renal function or of worsening of renal function in the patients with renal impairment were recorded." It is concluded that aescin has excellent tolerability in a clinical setting.
Raw Horse Chestnut seed, leaf, bark and flower are toxic due to the presence of esculin and should not be ingested. Horse chestnut seed is classified by the FDA as an unsafe herb. The glycoside and saponin constituents are considered toxic.
Aesculus hippocastanum is used in Bach flower remedies. When the buds are used it is referred to as "chestnut bud" and when the flowers are used it is referred to as "white chestnut".
Anne Frank Tree
A famous specimen of the horse-chestnut was the Anne Frank Tree in the centre of Amsterdam, which she mentioned in her diary and which survived until August 2010, when a heavy wind blew it over. Eleven young specimens, sprouted from seeds from this tree, were transported to the United States. After a long quarantine in Indianapolis, each tree was shipped off to a new home at a notable museum or institution in the United States, such as the 9/11 Memorial Park, Central H.S. in Little Rock, and two Holocaust Centers. One of them was planted outdoors in March 2013 in front of the Children's Museum of Indianapolis, where they were originally quarantined. 
- Bleeding canker. Half of all horse-chestnuts in Great Britain are now showing symptoms to some degree of this potentially lethal bacterial infection.
- Guignardia leaf blotch, caused by the fungus Guignardia aesculi
- Wood rotting fungi, e.g. such as Armillaria and Ganoderma
- Horse-chestnut scale, caused by the insect Pulvinaria regalis
- Horse-chestnut leaf miner, Cameraria ohridella, a leaf mining moth. also affecting large numbers of UK trees.
- Phytophthora bleeding canker, a fungal infection.
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- Horse Chestnut, Memorial Sloan-Kettering Cancer Center
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