There are 25 different species of Horse-Chestnut native to the temperate northern hempisphere. In North America they are referred to as "Buckeyes" and have 10 native species. Europe and Asia have 15 native species and they are reffered to as Horse-Chestnuts, some are also called "white Chestnuts" and "Red Chestnuts". In Britian they are sometimes called "Conker Trees" because of the relation with the game of Conkers.
The name Horse-chestnut, hyphenated here to avoid confusion with the true chestnuts (Castanea, Fagaceae), is also often given as "Horse Chestnut" or "Horsechestnut". One species very popular in cultivation, the Common Horse-chestnut Aesculus hippocastanum is also often known as just "Horse-chestnut". Linnaeus named the genus Aesculus after the Roman name for an edible acorn. The use of the term "horse" refers to their strength or inedibility, the word "horse" originally meant strong or powerful, and does not here refer to their fitness as fodder for horses, except in folk etymology. The name buckeye derives from the resemblance of the seed to the brown eye of a buck (male deer), and horse-chestnut from the external resemblance of the seed to a chestnut, but being inedible. The Buckeye blooms in summer and the Horse-chestnut in late spring.
Horse-chestnut is a woody plant from 4 to 35 m tall (depending on species), and has stout shoots with resinous, often sticky, buds; opposite, palmately divided leaves, often very large (to 65 cm across in the Japanese Horse-chestnut Aesculus turbinata); and showy insect-pollinated flowers, with a single four- or five-lobed petal (actually four or five petals fused at the base). Flowering starts after 80–110 growing degree days. The fruit is a rich glossy brown to blackish-brown nut 2–5 cm diameter, usually globose with one nut in a green or brown husk, but sometimes two nuts together in one husk, in which case the nuts are flat on one side; the point of attachment of the nut in the husk shows as a large circular whitish scar. The husk has scattered soft spines in some species, spineless in others, and splits into three sections to release the nut.
The nuts contain high concentrations of a saponin-class toxin called Aesculin, which is toxic to many animals including humans because it causes hemolysis (destruction of red blood cells). The saponin can be eliminated by leaching the pulverized nuts in multiple changes of boiling water, to yield a wholesome starchy porridge once important to some Native American tribes. Some animals, notably deer and squirrels, are resistant to the toxins and can eat the nuts directly. An interesting side-note is that Aesculin is a natural pH indicator which, when extracted turns from colourless to fluorescent blue under UV light in an acidic pH range.
Crushed buckeye nuts have also been thrown into lakes by poachers, to kill fish for easy capture. California Buckeyes Aesculus californica are known to cause poisoning of honeybees from toxic nectar (other locally native bee species not being affected). Other buckeye species are thought to have the same effect, but the toxins are diluted because the trees are not usually abundant enough in any one area. The wood is very pale whitish-brown, fairly soft and little-used. Uses include cheap furniture, boxes and firewood. In several European countries a new disease has been found in several species of Aesculus. For more information check http://www.kastanjeziekte.wur.nl (in Dutch). In Britain and Ireland the game of conkers remains a common childhood pastime. In some cultures, the buckeye tree is thought to bring good luck. The Mexican Buckeye is related to Aesculus, but is in a separate genus, Ungnadia.