The genus Aesculus (// or //) comprises 13–19 species of flowering plants in the soapberry and lychee family Sapindaceae. They are trees and shrubs native to the temperate Northern Hemisphere, with 6 species native to North America and 7–13 species native to Eurasia. There are also several hybrids. Aesculus exhibits a classical arcto-Tertiary distribution.[a]
Linnaeus named the genus Aesculus after the Roman name for an edible acorn. Common names for these trees include "buckeye" and "horse chestnut". Some are also called white chestnut or red chestnut. In Britain, they are sometimes called conker trees because of their link with the game of conkers, played with the seeds, also called conkers.
Aesculus species have stout shoots with resinous, often sticky, buds; opposite, palmately divided leaves, often very large—to 65 cm (26 in) across in the Japanese horse chestnut Aesculus turbinata. The seeds of the Aesculus are traditionally used in a game called conkers in Europe. Species are deciduous or evergreen. Flowers are showy, insect- or bird-pollinated, with four or five petals fused into a lobed corolla tube, arranged in a panicle inflorescence. Flowering starts after 80–110 growing degree days. The fruit matures to a capsule, 2–5 cm (25⁄32–1 31⁄32 in) diameter, usually globose, containing one to three seeds (often erroneously called a nut) per capsule. Capsules containing more than one seed result in flatness on one side of the seeds. The point of attachment of the seed in the capsule (hilum) shows as a large circular whitish scar. The capsule epidermis has "spines" (botanically: prickles) in some species, while other capsules are warty or smooth. At maturity, the capsule splits into three sections to release the seeds.
All parts of the buckeye or horse chestnut tree are moderately toxic, including the nut-like seeds. The toxin affects the gastrointestinal system, causing gastrointestinal disturbances. The USDA notes that the toxicity is due to saponin aescin and glucoside aesculin, with alkaloids possibly contributing. Native Americans used to crush the seeds and the resulting mash was thrown into still or sluggish waterbodies to stun or kill fish.  They would then boil and drain (leach) the fish at least three times in order to dilute the toxin's effects. New shoots from the seeds also have been known to kill grazing cattle.
The genus has traditionally been treated in the ditypic family Hippocastanaceae along with Billia, but recent phylogenetic analysis of morphological and molecular data has caused this family, along with the Aceraceae (Maples and Dipteronia), to be included in the soapberry family (Sapindaceae). The species of Aesculus include:
- Aesculus arguta: Aesculus glabra
- Aesculus californica: California buckeye (western North America)
- Aesculus × carnea: red horse chestnut
- Aesculus chinensis: Chinese horse chestnut (eastern Asia)
- Aesculus chinensis var. wilsonii: Wilson's horse chestnut (eastern Asia)
- Aesculus flava (A. octandra): yellow buckeye (eastern North America)
- Aesculus glabra: Ohio buckeye (eastern North America)
- Aesculus hippocastanum: common horse chestnut (Europe, native to the Balkans)
- Aesculus indica: Indian horse chestnut (eastern Asia)
- Aesculus neglecta: dwarf buckeye (eastern North America)
- Aesculus parviflora: bottlebrush buckeye (eastern North America)
- Aesculus parryi: Parry's buckeye (western North America, endemic in Baja California del Norte)
- Aesculus pavia: red buckeye (eastern North America)
- Aesculus pavia var. flavescens: Texas yellow buckeye, yellow woolly buckeye (eastern North America, narrowly endemic in Texas)
- Aesculus sylvatica: painted buckeye (eastern North America)
- Aesculus turbinata: Japanese horse chestnut (eastern Asia, endemic in Japan)
- Aesculus wangii: Aesculus assamica (eastern Asia)
The most familiar member of the genus worldwide is the common horse chestnut Aesculus hippocastanum. The yellow buckeye Aesculus flava (syn. A. octandra) is also a valuable ornamental tree with yellow flowers, but is less widely planted. Among the smaller species, the bottlebrush buckeye Aesculus parviflora also makes a very interesting and unusual flowering shrub. Several other members of the genus are used as ornamentals, and several horticultural hybrids have also been developed, most notably the red horse chestnut Aesculus × carnea, a hybrid between A. hippocastanum and A. pavia.
Interpretations of the tree leaves can be seen in architectural details in the Reims Cathedral.
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- Aesculus glabra (Ohio buckeye) King's American Dispensatory
- Winter ID pictures