The taxonomy of the plant family Liliaceae has had a complex history since its first description in the mid-eighteenth century. Originally, the Liliaceae were defined as having a "calix" (perianth) of six equal-coloured parts, six stamens, a single style, and a superior, three-chambered (trilocular) ovary turning into a capsule fruit at maturity. The taxonomic circumscription of the family Liliaceae progressively expanded until it became the largest plant family and also extremely diverse, being somewhat arbitrarily defined as all species of plants with six tepals and a superior ovary. It eventually came to encompass about 300 genera and 4,500 species, and was thus a "catch-all" and hence paraphyletic. Only since the more modern taxonomic systems developed by the Angiosperm Phylogeny Group (APG) and based on phylogenetic principles has it been possible to identify the many separate taxonomic groupings within the original family and redistribute them, leaving a relatively small core as the modern family Liliaceae, with fifteen genera and 600 species.
The Liliaceae emerged from the Liliales order, separating from its sister clade around 52 million years ago (mya), and diversifying around 34 mya. Of the major clades, the Lilieae arose in Eurasia while the Medeoleae arose in North America but was subsequently dispersed, as may have the Streptopoideae and Calochortoideae. Liliaceae fossils have been dated to the Paleogene and Cretaceous eras in the Antarctic.
The Liliaceae probably arose as shade plants, with subsequent evolution to open areas including deciduous forest in the more open autumnal period. This was accompanied by a shift from rhizomes to bulbs, to more showy flowers, the production of capsular fruit and narrower parallel-veined leaves.
While the suprageneric (above genus level) structure of the family has varied greatly with its ever-changing circumscription, as currently constituted the family consists of three subfamilies: Lilioideae, Calochortoideae and Streptopoideae. Lilioideae is further divided into two tribes, Medeoleae and Lilieae. The three subfamilies contain fifteen genera and approximately 600 species in all.
History
Pre-Darwinian The type genus, Lilium, from which the name of the family was derived, was originally formally described by Carl Linnaeus in 1753, with seven species. He placed Lilium within the Hexandria Monogynia (six stamens, one carpel) in his sexual classification in the Species Plantarum. The family Liliaceae was first described by Michel Adanson in 1763, but formally named by Antoine Laurent de Jussieu in 1789. Adanson described eight subfamilies with 78 genera, however the subfamily he described as Lis (lilies) had seven genera (Uvularia, Mithridatium, Mendoni, Lilium, Fritillaria, Imperialis — now part of Fritillaria — and Tulipa) of which four are in the modern genus. Jussieu placed these into the ordo, Lilia in the classis, Stamina Perigyna of the Monocotyledones (monocots), with eight genera (Tulipa, Erythronioum, Methonica, Uvularia, Fritillaria, Imperialis, Lilium, Yucca) only four of which remain in the family. He defined Lilia as "calix" (perianth) of six equal coloured parts, six stamens, a superior ovary, single style, and trilocular capsule. The term ordo at that time was closer to what we now understand as family, rather than order. Although Jussieu used the Latin "lilia" in his Genera Plantarum, elsewhere he used the French "liliacées", as had Adanson. The word "Liliaceae" was soon widely used by botanists such as Samuel Frederick Gray, John Lindley, and Pierre-Joseph Redouté in the early nineteenth century.
Gray (1821) provided the first description of Jussieu's scheme in English, identifying two genera occurring in Britain (Tulipa, Fritillaria), distinguished by the absence or presence of basal nectaries. His key used the presence of six equal stamens, a single style, a simple petaloid (undifferentiated tepals resembling petals) perianth and a trilocular capsule with flat seeds to identify the family. Although Augustin De Candolle (1813) had not explicitly described the Liliaceae, his overall classification scheme influenced many later writers including Gray. In this scheme, the Liliaceae were considered a family within those vascular plants (Vasculares) whose vascular bundles were thought to arise from within (Endogènes, endogenous), a term he preferred to Monocotylédonés. Jussieu's Monocotyledones became the Phanérogames (Phenogamae in Gray), meaning "visible seed", hence Endogenæ phanerogamæ. Candolle also instituted the concept of ordered ranks, based on classes, subclasses, familles (Latin: ordines naturales) and tribus (tribes), subdividing the Liliaceae.
Lindley was the first post-Linnaean English systematist, publishing his work in 1830, and following the reasoning of Jussieu he used the term tribe to describe the Liliaceae as a division of the hexapetaloid monocots, characterised by a superior ovary, highly developed perianth, inward turning anthers, a trilocular polyspermous capsule and seeds with a soft spongy coat. He offered seven genera as examples (Erythronium, Lilium, Calochortus, Blandfordia, Polianthes, Hemerocallis and Funkia). By 1846, in his final work, he refined and greatly expanded his taxonomy, favouring the term Alliances of Endogens over monocots as a class, of which there were eleven. Of these "alliances", the Liliales consisted of four Orders (families) including Liliaceae, which he referred to as lilyworts in the vernacular, with 133 genera and 1200 species. In this work he unhappily acknowledged the confusing array of different approaches to the classification of the Liliaceae, the lack of a clear definition, and the great diversity in the circumscription of the order, which had expanded vastly, with many subdivisions. As he saw it, the Liliaceae had already become a ("catch-all") grouping, being "everything that does not belong to the other parts of the Lilial Alliance", but expressed hope that the future would reveal some characteristic that would group them better. In other words, he foresaw that Liliaceae would come to be regarded as paraphyletic. By the time of the next major British classification, that of Bentham and Hooker in 1883 (published in Latin) several of Lindley's other families had been absorbed into the Liliaceae. This was the last major classification using the "natural" or pre-evolutionary approach to classification, based on characteristics selected a posteriori in order to group together taxa that have the greatest number of shared characteristics. This approach, also referred to as polythetic was superseded by ones based on an understanding of the acquisition of characteristics through evolution, referred to as phyletic.
Post-Darwinian
Although Charles Darwin's Origin of Species (1859) preceded Bentham and Hooker's publication, the latter project was commenced much earlier and George Bentham was initially sceptical of Darwinism. The new phyletic approach changed the way that taxonomists considered plant classification, incorporating evolutionary information into their schemata, but this did little to further define the circumscription of Liliaceae. The major works in the late nineteenth and early twentieth century employing this approach were in the German literature, the Eichler (1875–1886), Engler and Prantl (1886–1924) and Wettstein (1901–1935) systems. These placed the Liliaceae into one of the major subdivisions of the monocotyledons, the Liliiflorae. In the English literature, Charles Bessey (1915) followed Adolf Engler in defining Liliaceae as "Pistil mostly 3-celled; stamens 6; perianth of two similar whorls, each of three similar leaves", although placing the Liliales in a novel subclass of monocots, the Strobiloideae, while from John Hutchinson (1959) onwards the Liliaceae were treated as part of the Liliales (see Table 1).
Over time the Liliaceae became increasingly broadly, and somewhat arbitrarily, defined as all species of plants with six tepals and a superior ovary. They eventually came to encompass about 300 genera and 4,500 species, within the order Liliales in the scheme of Arthur Cronquist (1981). Cronquist was a "lumper" preferring a small number of very large groupings and his classification of the Liliaceae represented the greatest expansion of the family to date. Cronquist placed most flowering petaloid monocots with six stamens in this very broad (and clearly polyphyletic) family, hence the alternative name lilioid monocots. He rejected the importance of ovary position and thus included with the Liliaceae with their superior ovary (hypogynous), the Amaryllidaceae, some species of which had an inferior ovary (epigynous), and which others separated into a distinct family. The Liliaceae were one of the major families in the Cronquist system which included 22 families in addition to Liliaceae in the more restricted sense (sensu stricto, s.s.) used by others. Later more conventional schemes include the system of Robert F. Thorne and that of Armen Takhtajan which characterised the family as petaloid monocots, characterised by showy flowers with tepals and without starch in the endosperm.
Deconstructing Liliaceae
Other botanists in the twentieth century echoed Lindley's concerns about the lack of a clearly defined grouping for Liliaceae. The earliest of these was Johannes Paulus Lotsy (1911), building on Wettstein's work. Lotsy suggested four separate families, Liliaceae, Alliaceae, Agapanthaceae and Gilliesiaceae, within the Liliifloren. This recognised the major groupings that would later be transferred to Amaryllidaceae as subfamilies Allioideae and Agapanthoideae, with Gilliesieae as a tribe within the Allioideae. This approach was later followed by Herbert Huber in 1969. These various proposals to separate small groups of genera into more homogeneous families made little impact until Rolf Dahlgren (1985), following Huber's lead, developed a system incorporating new information, including synapomorphic characters (i.e., shared characters believed to have evolved from a common ancestor). While Cronquist was a "lumper", Dahlgren was a "splitter", preferring a larger number of more homogeneous groupings. Where Cronquist saw one family, Dahlgren saw forty distributed over three orders (predominantly Liliales and Asparagales), reducing Liliaceae to ten genera (see Table 3). Over the 1980s, in the context of a more general review of the classification of angiosperms, the Liliaceae were subjected to more intense scrutiny. By the end of that decade, the Royal Botanic Gardens at Kew, the British Museum of Natural History and the Edinburgh Botanical Gardens formed a committee to examine the possibility of separating the family into smaller taxa, at least for the purpose of organizing their herbaria. That committee finally recommended that 24 new families be created in the place of the original broad Liliaceae, largely by elevating subfamilies to the rank of separate families.