Leitneria Chapm.

A genus of shrubs or small trees from Southeastern USA that has puzzled taxonomists since Leitneria floridana was first described in 1860. It was a monospecific genus until 2011, when the plants in the western disjunct populations were referred to a second species, L. pilosa, in which two subspecies were also recognised.

In addition to uncertainty about specific identifications, the genus has had a mobile history at higher rank. Chapman (1860) placed it in Myricaceae (currently Fagales, at the time considered close to Salicaceae) and De Candolle (1864) followed suit, but the genus was later placed in its own family, Leitneriaceae (Bentham & Hooker 1865), and even its own order, Leitneriales (Engler & Prantl (eds) 1897). It subsequently embarked on a whistle-stop tour of orders, as taxonomists struggled to pin down its relationship to other genera. For Solereder (1908), the order showed affinity to Dipterocarpaceae (Malvales), for Cronquist (1981) and Takhtajan (1980), it belonged in Hamamelidales, for Dahlgren (1980), Sapindales, for Thorne (1981), Rutales. In 1983 Petersen and Fairbrothers found that serological evidence supported placement within Sapindales, on the basis of elevated reactivities of pollen protein extracts of Leitneria with taxa in Simaroubaceae; in 1995, Fernando et al. confirmed placement in Simaroubaceae based on molecular analyses. Tobe (2011) found that embryological evidence further supported the transfer to Simaroubaceae, and that the genus is closely related to the tropical Brucea, Soulamea, and Amaroria. A molecular phylogeny based on chloroplast and nuclear markers proposed by Clayton et al. (2007) confirmed that Leitneria is sister to a clade of those three genera. The Simaroubaceae is also known as the quassia or tree-of-heaven family, alluding to its best-known member, Tree of Heaven (Ailanthus altissima).

While attention was focused on higher-level systematics of Leitneria, the infrageneric systematics were for a long time neglected. Several authors had suggested that the disjunct populations of what was known as L. floridana (in Florida, Georgia, Arkansas, Missouri and Texas) perhaps held more than one species. Oliver, as early as 1867, had indicated that a specimen collected by Drummond in Texas in 1835 might represent a second species. Channel and Wood (1962) noted that the species was variable in height, leaf shape, length of catkins and fruit size, but that no formal taxonomic segregates had been proposed. In 2011, Schrader and Graves performed molecular and morphometric analyses on samples from populations in all five states where it is found. They found that the western populations (Missouri, Arkansas, Texas) were sufficiently distinct to merit separation into a new species, L. pilosa, and that within this new species, populations in Missouri and Arkansas (subsp. ozarkana) differed from those in Texas (subsp. pilosa), to an extent that justified infraspecific separation. The differences between these taxa are summarised in the key below.

Leitneria once had a broader distribution including Eurasia. The fossil record indicates it was a rare element in the late Cenozoic floras of Italy (Martinetto 2001). In North America, it is plausible that it was among the first taxa to colonise the Southeastern Coastal plain, a hypothesis supported by the presence of fossil Leitneria in the Eocene (55–34 million years ago) (Brown 1960). This early presence and the fact that Leitneria is deciduous and cold hardy well beyond its range suggest that it invaded the Coastal Plain from a northern temperate region (Sharma, Schrader & Graves 2008).

Today the genus is found in disjunct populations with significant geographical separation, suggesting that the populations have been genetically isolated and that infrageneric divergence has occurred. The plants reproduce by seed and by rhizomes, but pollen dispersed by wind is unlikely to be transported between populations. Fruits are dispersed by water, where they float for up to two months, but dispersal is restricted because colonies are found in swamps or waterways of limited size. Animals may also help disperse the fruit, but dispersal would be limited to the summer home ranges of local vertebrates, which do not exceed 20 km. The natural genetic exchange between colonies of Leitneria is therefore highly unlikely at distances over 80 km, according to Schrader and Graves (2011). The combination of disjunct isolation, genetic differentiation and morphological characters warrants taxonomic distinction of the three populations (Weakley & Southeastern Flora Team 2025), though Yatskievych (2013) was sceptical that they were reliably distinct based on the morphological characters presented by Schrader and Graves (2011). According to Ron Lance (pers. comm. 2025), the horticultural community is ‘unenthused’ by the divisions because of difficulties of applying the morphological distinctions to cultivated specimens. As plants propagated in cultivation may have originated from male and female plants of different geographical origin, they do not key out easily based on leaf shape, size and vestiture. The most consistent trait that can be used to distinguish L. floridana is the glabrous or near glabrous upper leaf surface. Provenance proves more important than morphology when determining the identity of plants.

It appears that the plants introduced to cultivation in 1894, when B.J. Bush sent material found in Missouri to the Arnold, would correspond to what is now L. pilosa subsp. ozarkana, and that this was also the taxon introduced to Kew in 1910 (see L. floridana species article). However, little mention is made of L. pilosa in cultivation, and most plants are still referred to as L. floridana. Here we discuss cultivation of all Leitneria taxa under L. floridana, as it is difficult to determine the identity of plants in cultivation at a species or subspecies level.

The genus was first described in 1860, in Chapman’s Flora of the Southeastern United States (Chapman 1860). According to Sargent (1895), Chapman had found it in 1847 near Apalachicola, Florida. However, it had been discovered over a decade earlier by Thomas Drummond, who collected a specimen by the Brazos River, Texas, in 1835 and sent it back to William J. Hooker at Kew (Hooker labelled the specimen but did not describe it; it would be what is now L. pilosa subsp. pilosa) (Oliver & Hooker 1867; Sargent 1895). But Leitneria had probably been first seen by E.F. Leitner during his explorations of the Everglades and east Florida in 1833 or in later expeditions prior to his death in 1838. Leitner did not describe the plants he found, and his herbarium specimens have been lost, the bulk of them when Berlin was bombed in 1943. So there seems to be no evidence that Leitner discovered the plant, though Gifford (1972) refers to Leitneria as ‘a mysterious plant discovered by Leitner,’ and Koller (1997) also implies Leitner found it. In his publication, Chapman makes no mention of Leitner, but the fact that he named the genus in his honour suggests that he may have known that he had discovered it.

Leitner was born Friedrich August Ludwig Leitner in Stuttgart in 1812, to a family of gardeners, then in 1831 migrated to the United States, where he took on the names Edward Fredrick. He settled in Charleston, South Carolina, where he registered as a student and gave lessons in classics, German, and botany, this last to a group that included women, which was a novelty at the time. He persuaded wealthy Charlestonians and others to sponsor him on an expedition to Florida in 1833. On his return to Charleston, he trained as a surgeon and accompanied military expeditions to Florida in that capacity, as a way of continuing botanical explorations, which had become dangerous due to conflicts with the local Seminole people. In 1838, while on one of those expeditions, he was mortally wounded (Gifford 1972). It is said he urged those who came to help him to look after their own safety and not risk their lives for him, as he could not live long. According to a contemporary account from a fellow combatant, heard from notoriously mendacious Seminole prisoners, the Seminoles found him and, on realising he was a medic, kept him alive in order to profit from his services, bearing him into the interior on a litter. Later, however, a vengeful Seminole, who had lost his brother in the fighting, ‘rushed upon him with a yell, and plunged his knife into his heart’ (Motte 1953). His body was never found. Had he avoided this dreadful demise, he might have contributed significantly to the botany of southern Florida. According to James Rhette Motte, who had studied with him in Charlotte, he was ‘a man of rare ability and singular modesty; excelling in the characteristics of the German scholar, and enthusiastically devoted to the sciences; of which botany was his favourite branch… [D]eath suddenly terminated his career of usefulness: depriving science of one of her indefatigable votaries; and mankind of the benefits of his laborious researches’ (Motte 1953).