For genera, see article.
As defined by the Angiosperm Phylogeny Group in 2009 (the APG III system), the family consists of three subfamilies: Asphodeloideae, Hemerocallidoideae and Xanthorrhoeoideae. Earlier these three had been treated as separate families, with the Xanthorrhoeaceae sensu stricto consisting only of the genus Xanthorrhoea.
The family has a wide, but scattered distribution throughout the tropics and temperate zones. Many of the species are cultivated as ornamentals. A few are grown commercially for cut flowers. Two species of Aloe are grown for their leaf sap, which has medicinal and cosmetic uses. Xanthorrhoea is endemic to Australia.
In some of the older systems of plant taxonomy, such as the Cronquist system, the plants that now form the family Dasypogonaceae were also considered to belong to this family. Molecular phylogenetic studies have shown that Dasypogonaceae belongs to the commelinids and is therefore not even in the same order as Xanthorrhoeaceae.
Members of the Xanthorrhoeaceae are diverse, with few characters uniting the three subfamilies currently recognized. The presence of anthraquinones is one common character. The flowers (the inflorescence) are typically borne on a leafless stalk (scape) which arises from a basal rosette of leaves. The individual flowers have jointed stalks (pedicels). A disk of woody tissue (a hypostase) is present at the base of the ovule.
The subfamily Xanthorrhoeoideae contains only the genus Xanthorrhoea, native to Australia. Plants typically develop thick woody stems; the flowers are arranged in a dense spike. Members of the subfamily Asphodeloideae are often leaf succulents, such as aloes and haworthias, although the subfamily also includes ornamental perennials such as red hot pokers (Kniphofia). Members of the subfamily Hemerocallidoideae are varied in habit. Daylilies (Hemerocallis) are one of the widely grown members of this subfamily.
The order Asparagales can be divided into a basal paraphyletic group, the "lower Asparagales", which includes the Xanthorrhoeaceae as defined here, and a well-supported monophyletic group of "core Asparagales", comprising Amaryllidaceae sensu lato and Asparagaceae sensu lato. Three separate families were at one time recognized (e.g. in the first APG system of 1998): Asphodelaceae, Hemerocallidaceae and Xanthorrhoeaceae. Molecular phylogenetic studies have shown that the three are closely related, although Rudall considered that the combination into a single clade was not supported by morphological analysis. The most recent APG classification, the APG III system of 2009, places the three former families into a single family, the Xanthorrhoeaceae sensu lato. The former families are treated as three subfamilies: Asphodeloideae, Hemerocallidoideae and Xanthorrhoeoideae.
The following phylogenetic tree for Xanthorrhoeaceae sensu lato is based on a molecular phylogenetic analysis of the DNA sequences of the chloroplast genes rbcL, matK, and ndhF. All branches have at least 70% bootstrap support. Of the 36 genera recognized by the authors, 29 were sampled. Eccremis was not sampled, but is added here because it is known to be closely related to Pasithea and is often combined with it. Hodgsoniola belongs somewhere in the grade from Tricoryne to Johnsonia. The unsampled genera, Astroloba, Chortolirion and Gasteria, belong to subfamily Asphodeloideae.
The family Xanthorrhoeaceae has had a complex history; its circumscription and placement in an order have varied widely. The expansion to include the subfamilies Asphodeloideae and Hemerocallidoideae first occurred as an option in the APG II system of 2003; earlier references to the Xanthorrhoeaceae relate only to the subfamily Xanthorrhoeoideae. The changes have been a consequence of improvement in molecular and morphological analysis and also a reflection of the increased emphasis on placing families within an appropriate order.
In the Cronquist system of 1981, members of the Xanthorrhoeaceae were placed in the order Liliales. Cronquist had difficulty classifying the less obviously delineated lilioid monocots; consequently, he placed taxa from both the modern orders Asparagales and Liliales into a single family Liliaceae.
The 1986 Flora of Australia was based on Cronquist's classification and placed 10 genera within Xanthorrhoeaceae. Bedford et al. acknowledged at the time that some authors, such as Dahlgren et al. (1985), were segregating the 10 genera into two or three separate families, rather than grouping them all under Xanthorrhoeaceae. A review of the systematics of the group, using anatomical and molecular data, led to the conclusion that four of the 10 genera should be placed in the family Dasypogonaceae and five in the family Lomandraceae, leaving only Xanthorrhoea in Xanthorrhoeaceae prior to the APG expansion. The only stable classification over time has been of the genus Xanthorrhoea, which has remained within Xanthorrhoeaceae. This single genus currently contains 30 species.
Subfamily Asphodeloideae Burnett
Subfamily Hemerocallidoideae Lindley
Subfamily Xanthorrhoeoideae M.W.Chase, Reveal & M.F.Fay
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