Halesia, as now defined, contains two species in North America. These are deciduous trees or shrubs with winter buds enclosed in scales. The leaves are alternate with serrate margins; stipules are absent. The white flowers occur in axillary fascicles or short racemes on one-year-old branchlets. Flowering occurs before the leaves emerge; pedicels are slender and jointed. The flowers are hermaphrodite, four-merous, with a tubular calyx, four-ribbed and opening into four teeth, a campanulate corolla, four- to five-lobed, and 7–16 stamens, in one series. The fruit is a dry, indehiscent drupe with two to four wings and a short beak (Fritsch 2009, Hwang & Grimes 1996).
The taxonomic position of the Chinese Perkinsiodendron macgregorii (formerly H. macgregorii) has recently been resolved (Fritsch et al. 2016). This species had previously been shown to be more closely related to the Asian genus Rehderodendron than to the American Halesia (Fritsch et al. 2001), and indeed in the field it can easily be mistaken for that genus (T. Hudson pers. comm. 2007), but it has recently been shown to be distinct from both and a new genus described. Perkinsiodendron shares with Halesia the character of winged fruits, whilst with Rehderodendron it shares the character of having stamen of unequal length. It is included in the key below for completeness.
The genus Halesia was named in honour of Dr Stephen Hales a pioneer plant physiologist, who was born at Bekesbourne, in Kent, in 1671, died at Teddington in 1761, and is best known botanically for his publicaiton Vegetable Staticks (1727).
The Silverbells, especially forms of H. carolina, are justifiably popular garden trees, valued for their masses of pure white pendent flowers in early summer. Unfortunately their names are becoming very confused as different points of taxonomic view are presented. A recent revision of the genus at the California Academy of Sciences, San Francisco has concluded that there are only two species in North America, H. carolina and H. diptera, the name H. carolina being applied to a single variable taxon that subsumes the familiar names H. tetraptera, H. monticola and H. parviflora, and the various combinations under which these epithets have appeared (Fritsch & Lucas 2000). This position is used in the Flora of North America account of the genus and is adopted in our cross-referencing. Earlier work by Reveal & Seldin (1976) also recognises two species, but admits that there is a case for recognising four. In this scenario all taxa are maintained as separate species or varieties, the name H. carolina being restricted to the small-flowered southern variants also known as H. parviflora. This viewpoint has been embraced by American horticulturists, and was given wide publicity in an article by Rick Darke (2006), but has not been adopted in Europe. Peter Fritsch (pers. comm. 2007) recommends the use of cultivar group names to denote horticulturally recognisable entities within the H. carolina complex (for example, H. carolina Monticola Group). It seems certain that the nomenclature of these trees in horticulture will be muddled for years to come. Halesia require moist, fertile, acidic soil and flourish best in areas with hot summers.
|1a||Stamens 8, subequal in length (4 long 4 short). Stamen, protrude beyond petals.||Perkinsiodendron macgregorii|
|1b||Stamens 7 to 16, equal in length. Stamen do not protrude beyond petals.||(2)|
|2a||Bark of juvenile twigs and stems with white epidermis. Corolla glabrous, fused almost to tip or at least more than half length; stamens 12–16; styles glabrous; fruits four-winged, wings roughly equal.||Halesia carolina|
|2b||Bark of juvenile twigs and stems without white fissures. Corolla hairy, free almost to base; stamens 7–10; styles hairy; fruits two- or four-winged, if four-winged then two broad wings alternate with two much narrower wings||Halesia diptera|