SAPINDACEAE (Soapberry Family)

Plants mostly monoecious or dioecious, often incompletely so, variously herbs, shrubs, or trees, occasionally vining. Leaves alternate or opposite, petiolate, the petiole bases usually noticeably swollen or expanded. Stipules mostly lacking (often present in Cardiospermum, occasionally partly free from the petiole base in Acer), but small tufts of purplish cobwebby hairs or minute dark scales often present at the leaflet bases in Aesculus. Leaf blades variously palmately, pinnately, or ternately compound or lobed, the leaflets of various shapes, the margins usually toothed or lobed. Inflorescences terminal or appearing lateral, panicles or clusters, usually lacking bracts and bractlets. Flowers actinomorphic or zygomorphic, hypogynous (the staminate ones often perigynous in Acer), mostly imperfect (a small number of perfect flowers often present in some inflorescences. Calyces of 4 or 5(6) sepals, these sometimes fused, often colored, variously shaped, sometimes persistent at fruiting. Corollas absent or of 4 or 5(6) free petals. Stamens 3–8 (reduced to tiny rudiments in pistillate flowers or appearing short but fully formed in Koelreuteria), sometimes exserted, the filaments free, usually attached to the inside of a sometimes inconspicuous nectar-ring, the anthers usually more or less exserted, attached at the base. Pistil 1 per flower (usually reduced to a peglike rudiment in staminate flowers), superior, of 2 or 3 fused carpels. Ovary usually with 2 or 3 locules, sometimes strongly lobed or flattened, the placentation axile. Styles 1 (then sometimes deeply 2-lobed) or 2 per flower, each (or each branch) with 1 stigma, this entire (capitate to club-shaped) or deeply 3-lobed. Ovules 1 or 2 per locule. Fruits samaras, drupelike berries, or capsules, if capsules then dehiscent longitudinally, 1–3-seeded. Seeds various, often with arils. About 130 genera, 1,450–1,900 species, nearly worldwide, most diverse in tropical regions.

Recent morphological and molecular studies have suggested that genera traditionally classified in the families Aceraceae (2 genera, about 115 species) and Hippocastanaceae (2 or 3 genera, 16–20 species) are better treated within an expanded circumscription of the Sapindaceae (Judd et al., 1994; Harrington et al., 2005; Buerki et al., 2009; Harris et al., 2009). Although the three families have traditionally been thought to be closely related (Cronquist, 1981, 1991), their combination adds further variation to an already morphologically variable family. Molecular phylogenetic research (Harrington et al., 2005; Buerki et al., 2009) has supported the close relationship between Aceraceae and Hippocastanaceae near the base of the lineage that includes the remaining genera of Sapindaceae. However, these studies have agreed that at the base of the entire lineage (below Aceraceae and Hippocastanaceae) is the odd, monospecific, Chinese genus Xanthoceras Bunge, which traditionally has always been classified in the Sapindaceae. The alternative approach of segregating Xanthoceras into its own family and resplitting the Aceraceae and Hippocastanaceae has also been proposed (Buerki et al., 2010), but has not yet met with widespread acceptance by botanists. The family Aceraceae was treated in Volume 2 of the present work (Yatskievych, 2006). The Missouri treatment of the maples was completed before most of the compelling molecular work on the group was published. Because of this, users of the present work will have to consult Volume 2 for the treatment of the genus Acer, which most botanists now agree should be included in a broadly circumscribed Sapindaceae. For convenience, the family description above includes those characters unique to Acer and the genus is included in the key to genera of Sapindaceae below. Aesculus, which was treated in the Hippocastanaceae by Steyermark (1963), is here included in the Sapindaceae.

The Sapindaceae include a number of tropical timber trees. Many of the species produce saponins and nonprotein amino acids that render them toxic to mammals, but surprisingly a number of the tropical genera produce edible fruits, several of which are commercially cultivated, including Blighia K.D. Koenig (ackee, akee), Dimocarpus Lour. (longan), Litchi Sonn. (litchi, lychee), Nephelium L. (rambutan), and Paullinia L. (guaraná). Often the edible portion is a fleshy aril surrounding the seed. Some of the edible species also are sold as juices. The hard seeds of some species are used as beads in handcrafts. The saponins of some members of the family have been extracted to make soap. However, in the United States, the main economic benefit from the family is in the variety of horticulturally important species, including maples, buckeyes (horse chestnuts), golden rain tree, and soapberry.

Steyermark (1963) included another member of the order Sapindales, Melia azedarach L. (Meliaceae; chinaberry tree, china tree, pride of India), in the flora without any definite records, stating that because the species was commonly cultivated in southeastern Missouri it likely would be discovered as an escape along a railroad or in a waste area. Since that time, no specimens have been collected in the state and the species is no longer very commonly grown as an ornamental shade tree in the region. It has been recorded from adjacent counties in northeastern Arkansas, but for the present must be excluded from formal treatment in the Missouri flora. Melia azederach is a tree to 15 m tall with large leaves that are 2-times pinnately compound and similar in appearance to those of Koelreuteria paniculata. The fruits (which contain poisonous tetranorterpenes known as meliatoxins) are superficially similar in size and appearance to those of Sapindus saponaria, but are drupes with a 5-locular stone rather than 1-seeded berries. The flowers of Melia differ from all members of the Sapindaceae in their unusual staminal structure: the filaments form a slender, dark purple tube terminating in 20–24 sharply pointed teeth, and the yellow anthers are attached inside the tube below the apex. The flowers of Melia also differ from those of Missouri members of the Sapindaceae in their 5(6) petals, which are 8–11 mm long, slender, stiffly spreading, and lilac to pale pinkish purple.