| Asphodeloideae | |
|---|---|
| Yellow Asphodel (Asphodeline lutea) | |
| Scientific classification | |
| Kingdom: | Plantae |
| clade: | Angiosperms |
| clade: | Monocots |
| Order: | Asparagales |
| Family: | Xanthorrhoeaceae |
| Subfamily: | Asphodeloideae |
| genera | |
|
see text |
|
Asphodeloideae is a subfamily of the monocot family Xanthorrhoeaceae in the order Asparagales. It has previously been treated as a separate family, Asphodelaceae.[1] The subfamily name is derived from the generic name of the type genus, Asphodelus. Members of group are native to Africa, central and western Europe, the Mediterranean basin, Central Asia and Australia, with one genus (Bulbinella) having some of its species in New Zealand. The greatest diversity occurs in South Africa.
The genera Aloe, Asphodelus and Kniphofia are perhaps the best known genera from their use in horticulture as ornamental plants.
Contents |
Asphodeloideae is distinguished by a general presence of anthraquinones, simultaneous microsporogenesis, atypical ovules morphology, and the presence of an aril.[2] Asphodeloideae also has a characteristic secondary growth by means of a secondary thickening meristem.[2] This character, however, is also found in other taxa in the Asparagales, including Agavaceae, Iridaceae, and Xanthorrhoeoideae. It is confined to Asparagales among the monocots and is believed to have evolved independently in most families.[2]
There is agreement among many researchers that Asphodeloideae can be further divided into a monophyletic group, Alooideae, and a non-monophyletic group of the remaining genera.[3][2][4] Alooideae are mainly rosulate leaf succulents, while the other genera are non-succulent. The genera in Alooideae are centered in southern Africa, while the other genera have mainly a Eurasian distribution.[4]
Alooideae comprises Aloe, Astroloba, Chamaealoe, Gasteria, Haworthia, Lomatophyllum, and Poellnitzia.[2] The genera within Alooideae have several morphological characters that can be readily distinguished in the field, namely the arrangement and type of leaf and inflorescence.
Alooidae leaves are arranged in strongly tufted terminal (in arborescent species, as in Aloe barberae) or basal rosettes and are arranged in distinct ranks. Leaves are succulent and have distinctive white or concolorous tubercules. This is hypothesized to be a derived condition, possibly as a mechanical defensive mechanism to make the leaf less palatable, or to prevent heat damage in arid conditions. As well, the cross-section of the leaves are distinctly shaped in a cymbiform or crescent-shape, which represents a synapomorphy for all Alooideae taxa.[3]
The inflorescence is compacted into a many-flowered spike, or a simple or branched raceme, and is apical although can seem axillary. Stems are monopodial until an inflorescence is formed, and then it becomes sympodial; this prevents the rosette from dying as in Agave. Flowers of all taxa within Alooideae are tubular and have a fusion of petaline tepals, although the amount of fusion varies among genera.[3]
Evidence for monophyly of Alooideae is based on distinctive karyotype and characteristic leaf morphology. The Bulbine group has characteristics of Alooideae, but is not included in the group due to a lack of tubular flowers. Kniphofia is considered an outgroup of Alooideae since it has tubular flowers and a fusion of perianth segments, but it lacks succulent leaves.[3]
The presence of aloin cells are a distinctive character of the Alooideae. These cells are present in all Alooideae but are absent in most of the other genera within Asphodeloideae.[2] There is a well-developed cap of thin-walled parenchyma cells at the phloem pole of each vascular bundle.[3] Chase posits that they are involved in secondary metabolite production,[2] but Beaumont suggests that the cells act as a storage tissue for compounds that are synthesized in the surrounding layer of cells.[5] The aloin cells are said to produce the characteristic thick exudate that is produced when the succulent leaf of Aloe is severed.[5] The aloin cells produce anthraquinone and chromone derivatives,[5] which may be responsible for the medicinal attributes of Aloe.
According to the AP-website as of May 2011[update], the subfamily includes some 15 genera in up to 800 species. The genera are listed below.[6] Other treatments combine some of these genera into as few as seven.
For a phylogeny of the family see the phylogenetic tree at Xanthorrhoeaceae. [7]