Xanthorrhoeaceae

Xanthorrhoeaceae
Xanthorrhoea preissii
Scientific classification
Kingdom: Plantae (unranked): Angiosperms (unranked): Monocots Order: Asparagales Family: Xanthorrhoeaceae Dumort.
Genera
See text
Distribution of Xanthorhoeaceae s. str.

Xanthorrhoeaceae is the botanical name of a family of flowering plants. Such a family has been recognized by most taxonomists, but the circumscription of the family has varied wildly.

The APG II system also recognizes this family and places the family in the order Asparagales, in the clade monocots and allows two options:

• sensu lato: the family consists not only of the genus Xanthorrhoea but also of the plants that otherwise would be in families Asphodelaceae and Hemerocallidaceae.
• sensu stricto: the family consists only of the genus Xanthorrhoea. In this option, the families Asphodelaceae and Hemerocallidaceae are recognized as separate, satellite families.

In previous systems of plant taxonomy the plants that now form the family Dasypogonaceae were also reckoned to belong to this family.

Phylogeny

The classification of Xanthorrhoeaceae has in the past included ten genera, listed in the Flora of Australia by Bedford et al. ^[1]. This has been reduced over time to currently just one genus, Xanthorrhoea; while the other nine genera were placed into other families such as Laxmanniaceae and Dasypogonaceae, these remained in the lilioid order Asparagales, along with Xanthorrhoeaceae.

Molecular DNA sequencing has uncovered several monocot clades^[2], although there still remains an unresolved polytomy between three groups: the lilioid orders, the commelinid clade, and the family Petrosavaceae. The main taxonomic issue within the Asparagales is the placement of Orchidaceae. Recent research that indicates Orchidaceae to be monophyletic^[3][2]. This places it as a sister taxon to all Asparagales, notwithstanding research such as Rudall’s morphological analysis^[4], which suggested a close relationship between Orchidaceae and certain other families within Asparagales, such as Hypoxidaceae.

The order Asparagales is divided into two groups. The lower grade contains early-diverging clades characterised by simultaneous microsporogenesis; and the higher clade is characterised by successive microsporogenesis^[5]. The morphological division between successive and simultaneous microsporogenesis is supported by through molecular analysis in comparing monocot rbcL nucleotide sequences, and plastid trnL-F sequences^[5]. The Angiosperm Phylogeny Website^[3] disputes this finding, claiming that placement based on changes in microsporogenesis is unclear. In the lower Asparagales, which has mostly simultaneous microsporogenesis, the successive type is present in Xanthorrhoeaceae, Hypoxidaceae, Iridaceae and Orchidaceae^[3]. This change has been described as a reversal from simultaneous to successive^[5], since this particular change is more frequent across the angiosperms. Even with these findings, simultaneous microsporogenesis continues to be of phylogenic importance within Asparagales^[6]. Rudall^[4] carried out a morphological cladistic analysis on Asparagales, which included an out-group sample of five families of Liliales. Rudall recognized that restrictions of this type of analysis depends on the broadness of out-group sampling, and the limits of the number of taxa sampled, especially when the lilioid orders have a high homoplasy of floral and vegetative characteristics. Regardless of these sampling problems, Rudall’s morphological analysis indicates that Asparagales has a monophyletic lower clade, rather than a grade. Rudall confirms Chase et al.’s^[5] molecular analysis by placing Xanthorrhoeaceae into the lower group in Asparagales, despite sharing the characteristic of successive microsporogenesis with the higher clade. Xanthorrhoeaceae has been found to be closely related to Asphodelaceae and Hemerocallidaceae, with Xanthorrhoeaceae and Asphodelaceae as sister taxa^[5][3]. There is some uncertainty in the branch of Hemerocallidaceae and the Xanthorrhoeaeace/Asphodelaceae branch, reflected in the 50–79% branch support in the phylogenetic relationships within Asparagales^[3]. Rudall^[4], in confirmation of this uncertainty, states that by placing Xanthorrhoeaceae in a clade with Asphodelaceae and Hemerocallidaceae, although supported by molecular analysis, is not supported by morphological analysis; however, Rudall does concede that it shares a thickening meristem with Asphodelaceae.

In order to resolve ths uncertainty between Xanthorhoeaceae, Asphodlaceae and Hemerocallidaceae, it has been suggested that the family Xanthorhoeaceae is to contain a scheme of three subfamilies: Asphodeloideae, Hemerocallidoideae and Xanthorrhoeoideae.^[7]

Classification

The family Xanthorrhoeaceae has been placed into four different orders over time. According to Rudall^[4] this re-classification has been a consequence of improvement in molecular and morphological analysis. Re-classification is also a reflection of the increased emphasis on placing families within an appropriate order.^[8] Early classification of Xanthorrhoeaceae placed the family in the order Liliaceae, followed by placement in the order Agavales, alongside the family Agavaceae. Later classification by Cronquist^[9] and Beadle^[10] followed more traditional methods.^[5] This classification included Xanthorrhoeaceae within the order Liliales, under the subclass Liliidae. Cronquist had difficulty classifying the less obviously delineated Lilioid monocots; consequently, taxa from both orders Asparagales and Liliales were placed into a single family Liliaceae by Cronquist.^[4] Flora of Australia replicated Cronquist's classification, by placing ten genera within Xanthorrhoeaceae.^[1] Bedford et al. acknowledged at this time that some authors, such as Dahlgren et al. (1985), were segregating the ten genera into two or three separate families, rather than all grouped under Xanthorrhoeaceae. A review of the systematics of the group, using anatomical and molecular data has led to the conclusion that the genera Baxteria, Calectasia, Dasypogon and Kingia should be included in the family Dasypogonaceae, based on the presence of silica bodies found in the epidermal cells of these genera^[11]. They also found that Acanthocarpus, Chamaexeros, Lomandra, Romnalda and Xerolirion should be included in the family Lomandraceae, due to similar leaf anatomy. Xanthorrhoea was found to be placed correctly in Xanthorrhoeaceae, due to its isolated taxonomy.^[11] The classification of the genera contained within the group has changed in light of new molecular analysis (combined rbcL and trnL-F DNA analysis) and morphological analysis (leaf anatomy, silica within cells), as well as through reviewing previous published literature on Xanthorrhoeaceae, as carried out by researchers such as Rudall and Chase et al. The only stable classification over time has been of the genus Xanthorrhoea, which has remained within Xanthorrhoeaceae. This single genus currently contains 30 species.^[3]

Biology and Ecology

Xanthorrhoeaceae are easily recognisable, with a dense tuft of long narrow leaves terminating in a stout woody spike. The spike contains a dense inflorescence with small flowers and capsular fruits^[3]. The spike can reach 3m or more^[12], a height which would help seed dispersal. The endosperm of the seed is thick walled^[3], a possible evolutionary development due to Australia’s dry climate.

Xanthorrhoeaceae have sympodial growth, where inflorescences put a kink in the trunk. This occurs when a new growth tip is slightly offset from the trunk’s central axis. This is otherwise known as apical displacement. Sympodial growth occurs in most monocots^[5] Xanthorrhoeaceae has a secondary thickening meristem, which is one of the identifying characteristics unique to Asparagales ^[5]

The ovary is secondarily superior, which appears to be related to infra-locular septal nectaries, although the position of the ovary is a flexible characteristic in the Asparagales^[3]

Biogeography

Xanthorrhoeaceae are endemic to Australia, with distribution in all States and mainland Territories. Nearly all species occur in regions with rainfall greater than 250 mm per year. Many species have limited ranges, and are often found in isolated clumps ^[12]

References

External links

Wikimedia Commons has media related to: Xanthorrhoeaceae

• Xanthorrhoeaceae in L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval. Version: 9 March 2006. http://delta-intkey.com
• NCBI Taxonomy Browser
• links at CSDL, Texas