Thus although most species in the order are herbaceous, some no more than 15 cm high, there are a number of climbers (e.g., some species of Asparagus), as well as several genera forming trees (e.g. Agave, Cordyline, Yucca, Dracaena, Aloe ), which can exceed 10 m in height. Succulent genera occur in several families (e.g. Aloe).
Almost all species have a tight cluster of leaves (a rosette), either at the base of the plant or at the end of a more-or-less woody stem as with Yucca. In some cases the leaves are produced along the stem. The flowers are in the main not particularly distinctive, being of a general 'lily type', with six tepals, either free or fused from the base and up to six stamina. They are frequently clustered at the end of the plant stem.
The Asparagales are generally distinguished from the Liliales by the lack of markings on the tepals, the presence of septal nectaries in the ovaries, rather than the bases of the tepals or stamen filaments, and the presence of secondary growth. They are generally geophytes, but with linear leaves, and a lack of fine reticular venation.
The Seeds characteristically have the external epidermis either obliterated (in most species bearing fleshy fruit), or if present, have a layer of black carbonaceous phytomelanin in species with dry fruits (nuts). The inner part of the seed coat is generally collapsed, in contrast to Liliales whose seeds have a well developed outer epidermis, lack phytomelanin, and usually display a cellular inner layer.
The orders which have been separated from the old Liliales are difficult to characterize. No single morphological character appears to be diagnostic of the order Asparagales.The flowers of Asparagales are of a general type among the lilioid monocots. Compared to Liliales, they usually have plain tepals without markings in the form of dots. If nectaries are present, they are in the septa of the ovaries rather than at the base of the tepals or stamens.Those species which have relatively large dry seeds have a dark, crust-like (crustose) outer layer containing the pigment phytomelan. However, some species with hairy seeds (e.g. Eriospermum, family Asparagaceae S.l.), berries (e.g. Maianthemum, family Asparagaceae s.l.), or highly reduced seeds (e.g. orchids) lack this dark pigment in their seed coats. Phytomelan is not unique to Asparagales (i.e. it is not a synapomorphy) but it is common within the order and rare outside it. The inner portion of the seed coat is usually completely collapsed. In contrast, the morphologically similar seeds of Liliales have no phytomelan, and usually retain a cellular structure in the inner portion of the seed coat.Most monocots are unable to thicken their stems once they have formed, since they lack the cylindrical meristem present in other angiosperm groups. Asparagales have a method of secondary thickening which is otherwise only found inDioscorea (in the order Disoscoreales). In a process called 'anomalous secondary growth', they are able to create new vascular bundles around which thickening growth occurs. Agave, Yucca, Aloe, Dracaena, Nolina and Cordyline can become massive trees, albeit not of the height of the tallest dicots, and with less branching. Other genera in the order, such as Lomandra and Aphyllanthes, have the same type of secondary growth but confined to their underground stems.Microsporogenesis (part of pollen formation) distinguishes some members of Asparagales from Liliales. Microsporogenesis involves a cell dividing twice (meiotically) to form four daughter cells. There are two kinds of microsporogenesis: successive and simultaneous (although intermediates exist). In successive microsporogenesis, walls are laid down separating the daughter cells after each division. In simultaneous microsporogenesis, there is no wall formation until all four cell nuclei are present. Liliales all have successive microsporogenesis, which is thought to be the primitive condition in monocots. It seems that when the Asparagales first diverged they developed simultaneous microsporogenesis, which the 'lower' Asparagale families retain. However, the 'core' Asparagales (see #Phylogeny section) have reverted to successive microsporogenesis.The Asparagales appear to be unified by a mutation affecting their telomeres (a region of repetitive DNA at the end of a chromosome). The typical 'Arabidopsis-type' sequence of bases has been fully or partially replaced by other sequences, with the 'human-type' predominating.Other apomorphic characters of the order according to Stevens are: the presence of chelidonic acid, anthers longer than wide, tapetal cells bi- to tetra-nuclear, tegmen not persistent, endosperm helobial, and loss of mitochondrial gene sdh3.
As circumscribed within the Angiosperm Phylogeny Group system Asparagales is the largest order within the monocotyledons, with 14 families, 1,122 genera and about 25,000–42,000 species, thus accounting for about 50% of all monocots and 10–15% of the flowering plants (angiosperms). The attribution of botanical authority for the name Asparagales belongs to Johann Heinrich Friedrich Link (1767 – 1851) who coined the word 'Asparaginae' in 1829 for a higher order taxon that included Asparagus although Adanson and Jussieau had also done so earlier (see History). Earlier circumscriptions of Asparagales attributed the name to Bromhead (1838), who had been the first to use the term 'Asparagales'.
The type genus, Asparagus, from which the name of the order is derived, was described by Carl Linnaeus in 1753, with ten species. He placed Asparagus within the Hexandria Monogynia (six stamens, one carpel) in his sexual classification in the Species Plantarum. The majority of taxa now considered to constitute Asparagales have historically been placed within the very large and diverse family, Liliaceae. The Liliaceae family was first described by Michel Adanson in 1763, and in his taxonomic scheme he created eight sections within it, including the Asparagi with Asparagus and three other genera. The system of organising genera into families is generally credited to Antoine Laurent de Jussieu who formally described both the Liliaceae and the type family of Asparagales, the Asparagaceae, as Lilia and Asparagi, respectively, in 1789. Jussieu established the hierarchical system of taxonomy (phylogeny), placing Asparagus and related genera within a division of Monocotyledons, a class (III) of Stamina Perigynia and 'order' Asparagi, divided into three subfamilies. The use of the term Ordo (order) at that time was closer to what we now understand as Family, rather than Order. In creating his scheme he used a modified form of Linnaeus' sexual classification but using the respective topography of stamens to carpels rather than just their numbers. While De Jussieu's Stamina Perigynia also included a number of 'orders' that would eventually form families within the Asparagales such as the Asphodeli (Xanthorrhoeaceae), Narcissi (Amaryllidaceae) and Irides (Iridaceae), the remainder are now allocated to other orders. Jussieu's Asparagi soon came to be referred to as Asparagacées in the French literature (Latin: Asparagaceae). Meanwhile, the 'Narcissi' had been renamed as the 'Amaryllidées' (Amaryllideae) in 1805, by Jean Henri Jaume Saint-Hilaire, using Amaryllis as the type species rather than Narcissus, and thus has the authority attribution for Amaryllidaceae. In 1810 Brown proposed that a subgroup of Liliaceae be distinguished on the basis of the position of the ovaries and be referred to as Amaryllideae and in 1813 de Candolle described Liliacées Juss. and Amaryllidées Brown as two quite separate families.
The literature on the organisation of genera into families and higher ranks became available in the English language with Samuel Frederick Gray's A natural arrangement of British plants (1821). Gray used a combination of Linnaeus' sexual classification and Jussieu's natural classification to group together a number of families having in common six equal stamens, a single style and a perianth that was simple and petaloid, but did not use formal names for these higher ranks. Within the grouping he separated families by the characteristics of their fruit and seed. He treated groups of genera with these characteristics as separate families, such as Amaryllideae, Liliaceae, Asphodeleae and Asparageae.
The circumscription of Asparagales has been a source of difficulty for many botanists from the time of John Lindley (1846), the other important British taxonomist of the early nineteenth century. In his first taxonomic work, An Introduction to the Natural System of Botany (1830) he partly followed Jussieu by describing a subclass he called Endogenae, or Monocotyledonous Plants (preserving de Candolle's Endogenæ phanerogamæ) divided into two tribes, the Petaloidea and Glumaceae. He divided the former, often referred to as petaloid monocots, into 32 orders, including the Liliaceae (defined narrowly), but also most of the families considered to make up the Asparagales today, including the Amaryllideae.
By 1846, in his final scheme Lindley had greatly expanded and refined the treatment of the monocots, introducing both an intermediate ranking (Alliances) and tribes within orders (i.e. families). Lindley placed the Liliaceae within the Liliales, but saw it as a paraphyletic ("catch-all") family, being all Liliales not included in the other orders, but hoped that the future would reveal some characteristic that would group them better. The order Liliales was very large and had become a used to include almost all monocotyledons with colourful tepals and without starch in their endosperm (the lilioid monocots). The Liliales was difficult to divide into families because morphological characters were not present in patterns that clearly demarcated groups. This kept the Liliaceae separate from the Amaryllidaceae (Narcissales). Of these Liliaceae was divided into eleven tribes (with 133 genera) and Amaryllidaceae into four tribes (with 68 genera), yet both contained many genera that would eventually segregate to each other's contemporary orders (Liliales and Asparagales respectively). The Liliaceae would be reduced to a small 'core' represented by the Tulipae tribe, while large groups such Scilleae and Asparagae would become part of Asparagales either as part of the Amaryllidaceae or as separate families. While of the Amaryllidaceae, the Agaveae would be part of Asparagaceae but the Alstroemeriae would become a family within the Liliales.
The number of known genera (and species) continued to grow and 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. They used the term 'series' to indicate suprafamilial rank, with seven series of monocotyledons (including Glumaceae), but did not use Lindley's terms for these. However they did place the Liliaceous and Amaryllidaceous genera into separate series. The Liliaceae were placed in series Coronariae, while the Amaryllideae were placed in series Epigynae. The Liliaceae now consisted of twenty tribes (including Tulipeae, Scilleae and Asparageae), and the Amaryllideae of five (including Agaveae and Alstroemerieae). An important addition to the treatment of the Liliaceae was the recognition of the Allieae as a distinct tribe that would eventually find its way to the Asparagales as the Allioideae subfamily of the Amaryllidaceae.
The appearance of Charles Darwin's Origin of Species in 1859 changed the way that taxonomists considered plant classification, incorporating evolutionary information into their schemata. The Darwinian approach led to the concept of phylogeny (tree-like structure) in assembling classification systems, starting with Eichler. Eichler, having established a hierarchical system in which the flowering plants (angiosperms) were divided into monocotyledons and dicotyledons, further divided into former into seven orders. Within the Liliiflorae were seven families, including Liliaceae and Amaryllidaceae. Liliaceae included Allium and Ornithogalum (modern Allioideae) and Asparagus.
Engler, in his system developed Eichler's ideas into a much more elaborate scheme which he treated in a number of works including Die Natürlichen Pflanzenfamilien (Engler and Prantl 1888) and Syllabus der Pflanzenfamilien (1892–1924). In his treatment of Liliiflorae the Liliineae were a suborder which included both Liliaceae and Amaryllidaceae families. The Liliaceae had eight subfamilies and the Amaryllidaceae four. In this rearrangement of Liliaceae, with fewer subdivisions, the core Liliales were represented as subfamily Lilioideae (with Tulipae and Scilleae as tribes), the Asparagae were represented as Asparagoideae and the Allioideae was preserved, representing the alliaceous genera. Allieae, Agapantheae and Gilliesieae were the three tribes within this subfamily. In the Amaryllidacea, there was little change from Bentham and Hooker. A similar approach was adopted by Wettstein.
In the twentieth century the Wettstein system (1901–1935) placed many of the taxa in an order called 'Liliiflorae'. Next Johannes Paulus Lotsy (1911) proposed dividing the Liliiflorae into a number of smaller families including Asparagaceae. Then Herbert Huber (1969, 1977), following Lotsy's example, proposed that the Liliiflorae be split into four groups including the 'Asparagoid' Liliiflorae.
The widely used Cronquist system (1968–1988) used the very broadly defined order Liliales.
These various proposals to separate small groups of genera into more homogeneous families, made little impact till that of Dahlgren (1985) incorporating new information including synapomorphy. Dahlgren developed Huber's ideas further and popularised them, with a major deconstruction of existing families into smaller units. They created a new order, calling it Asparagales. This was one of five orders within the superorder Liliiflorae. Where Cronquist saw one family, Dahlgren saw forty distributed over three orders (predominantly Liliales and Asparagales). 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 at least for the organization of 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.
The order Asparagales as currently circumscribed has only recently been recognized in classification systems, through the advent of phylogenetics. The 1990s saw considerable progress in plant phylogeny and phylogenetic theory, enabling a phylogenetic tree to be constructed for all of the flowering plants. The establishment of major new clades necessitated a departure from the older but widely used classifications such as Cronquist and Thorne based largely on morphology rather than genetic data. This complicated discussion about plant evolution and necessitated a major restructuring. rbcL gene sequencing and cladistic analysis of monocots had redefined the Liliales in 1995. from four morphological orders sensu Dahlgren. The largest clade representing the Liliaceae, all previously included in Liliales, but including both the Calochortaceae and Liliaceae sensu Tamura. This redefined family, that became referred to as core Liliales, but corresponded to the emerging circumscription of the Angiosperm Phylogeny Group (1998).
The 2009 revision of the Angiosperm Phylogeny Group system, APG III, places the order in the clade monocots.
From the Dahlgren system of 1985 onwards, studies based mainly on morphology had identified the Asparagales as a distinct group, but had also included groups now located in Liliales, Pandanales and Zingiberales. Research in the 21st century has supported the monophyly of Asparagales, based on morphology, 18S rDNA, and other DNA sequences, although some phylogenetic reconstructions based on molecular data have suggested that Asparagales may be paraphyletic, with Orchidaceae separated from the rest. Within the monocots, Asparagales is the sister group of the commelinid clade.
This cladogram shows the placement of Asparagales within the orders of Lilianae sensu Chase & Reveal (monocots) based on molecular phylogenetic evidence. The lilioid monocot orders are bracketed, namely Petrosaviales, Dioscoreales, Pandanales, Liliales and Asparagales. These constitute a paraphyletic assemblage, that is groups with a common ancestor that do not include all direct descendants (in this case commelinids as the sister group to Asparagales); to form a clade, all the groups joined by thick lines would need to be included. While Acorales and Alismatales have been collectively referred to as "alismatid monocots" (basal or early branching monocots), the remaining clades (lilioid and commelinid monocots) have been referred to as the "core monocots". The relationship between the orders (with the exception of the two sister orders) is pectinate, that is diverging in succession from the line that leads to the commelinids. Numbers indicate crown group (most recent common ancestor of the sampled species of the clade of interest) divergence times in mya (million years ago).
A phylogenetic tree for the Asparagales, including those families that were reduced to subfamilies, is shown below. Of these, the two largest families are Amaryllidaceae and Asparagaceae.
The tree shown above can be divided into a basal paraphyletic group, the 'lower Asparagales (asparagoids)', from Orchidaceae to Xanthorrhoeaceae sensu lato, and a well-supported monophyletic group of 'core Asparagales' (higher asparagoids), comprising the two largest families, Amaryllidaceae sensu lato and Asparagaceae sensu lato.
Two differences between these two groups (although with exceptions) are: the mode of microsporogenesis and the position of the ovary. The 'lower Asparagales' typically have simultaneous microsporogenesis (i.e. cell walls develop only after both meiotic divisions), which appears to be an apomorphy within the monocots, whereas the 'core Asparagales' have reverted to successive microsporogenesis (i.e. cell walls develop after each division). The 'lower Asparagales' typically have an inferior ovary, whereas the 'core Asparagales' have reverted to a superior ovary. A 2002 morphological study by Rudall treated possessing an inferior ovary as a synapomorphy of the Asparagales, stating that reversions to a superior ovary in the 'core Asparagales' could be associated with the presence of nectaries below the ovaries. However, Stevens notes that superior ovaries are distributed among the 'lower Asparagales' in such a way that it is not clear where to place the evolution of different ovary morphologies. The position of the ovary seems a much more flexible character (here and in other angiosperms) than previously thought.
The APG III system when it was published in 2009, greatly expanded the families Xanthorrhoeaceae, Amaryllidaceae, and Asparagaceae. Thirteen of the families of the earlier APG II system were thereby reduced to subfamilies within these three families. The APG II families (left) and their equivalent APG III subfamilies (right) are as follows:
Orchidaceae is the largest family of all angiosperms and hence by far the largest in the order. The Dahlgren system recognized three families of orchids, but DNA sequence analysis later showed that these families are polyphyletic and so should be combined. Several studies suggest (with high bootstrap support) that Orchidaceae is the sister of the rest of the Asparagales. Other studies have placed the orchids differently in the phylogenetic tree, generally among the Boryaceae-Hypoxidaceae clade. The position of Orchidaceae shown above seems the best current hypothesis, but cannot be taken as confirmed.
Orchids have simultaneous microsporogenesis and inferior ovaries, two characters that are typical of the 'lower Asparagales'. However, their nectaries are rarely in the septa of the ovaries, and most orchids have dust-like seeds, atypical of the rest of the order. (Some members of Vanilloideae and Cypripedioideae have crustose seeds, probably associated with dispersal by birds and mammals that are attracted by fermenting fleshy fruit releasing fragrant compounds, e.g. vanilla.)
In terms of the number of species, Orchidaceae diversification is remarkable. However, although the other Asparagales may be less rich in species, they are more variable morphologically, including tree-like forms.
The four families excluding Boryaceae form a well-supported clade in studies based on DNA sequence analysis. All four contain relatively few species, and it has been suggested that they be combined into one family under the name Hypoxidaceae sensu lato. The relationship between Boryaceae (which includes only two genera, Borya and Alania), and other Asparagales has remained unclear for a long time. The Boryaceae are mycorrhizal, but not in the same way as orchids. Morphological studies have suggested a close relationship between Boryaceae and Blandfordiaceae. There is relatively low support for the position of Boryaceae in the tree shown above.
Ixioliriaceae to Xeronemataceae
The relationship shown between Ixioliriaceae and Tecophilaeaceae is still unclear. Some studies have supported a clade of these two families, others have not. The position of Doryanthaceae has also varied, with support for the position shown above, but also support for other positions.
The clade from Iridaceae upwards appears to have stronger support. All have some genetic characteristics in common, having lost Arabidopsis-type telomeres. Iridaceae is distinctive among the Asparagales in the unique structure of the inflorescence (a ripidium), the combination of an inferior ovary and three stamens, and the common occurrence of unifacial leaves whereas bifacial leaves are the norm in other Asparagales.
Members of the clade from Iridaceae upwards have infra-locular septal nectaries, which Rudall interpreted as a driver towards secondarily superior ovaries.
Xanthorrhoeaceae sensu lato + 'core Asparagales'
The next node in the tree (Xanthorrhoeaceae sensu lato + the 'core Asparagales') has strong support. 'Anomalous' secondary thickening occurs among this clade, e.g. in Xanthorrhoea (family Xanthorrhoeaceae sensu lato) and Dracaena (family Asparagaceae sensu lato), with species reaching tree-like proportions.
The 'core Asparagales', comprising Amaryllidaceae sensu lato and Asparagaceae sensu lato, are a strongly supported clade, as are clades for each of the families. Relationships within these broadly defined families appear less clear, particularly within the Asparagaceae sensu lato. Stevens notes that most of its subfamilies are difficult to recognize, and that significantly different divisions have been used in the past, so that the use of a broadly defined family to refer to the entire clade is justified. Thus the relationships among subfamilies shown above, based on APWeb as of December 2010, is somewhat uncertain.
Several studies have attempted to date the evolution of the Asparagales, based on phylogenetic evidence. Earlier studies generally give younger dates than more recent studies, which have been preferred in the table below.
A 2009 study suggests that the Asparagales have the highest diversification rate in the monocots, about the same as the order Poales, although in both orders the rate is little over half that of the eudicot order Lamiales, the clade with the highest rate.
The taxonomic diversity of the monocotyledons is described in detail by Kubitzki. Up-to-date information on the Asparagales can be found on the Angiosperm Phylogeny Website.
The APG III system's family circumscriptions are being used as the basis of the Kew-hosted World Checklist of Selected Plant Families. With this circumscription, the order consists of 14 families (Dahlgren had 31) with approximately 1120 genera and 26000 species.
Order Asparagales LinkFamily Amaryllidaceae J.St.-Hil. (including Agapanthaceae F.Voigt, Alliaceae Borkh.)Family Asparagaceae Juss. (including Agavaceae Dumort. [which includes Anemarrhenaceae, Anthericaceae, Behniaceae and Herreriaceae], Aphyllanthaceae Burnett, Hesperocallidaceae Traub, Hyacinthaceae Batsch ex Borkh., Laxmanniaceae Bubani, Ruscaceae M.Roem. [which includes Convallariaceae] and Themidaceae Salisb.)Family Asteliaceae Dumort.Family Blandfordiaceae R. Dahlgren & CliffordFamily Boryaceae M.W. Chase, Rudall & ConranFamily Doryanthaceae R. Dahlgren & CliffordFamily Hypoxidaceae R.Br.Family Iridaceae Juss.Family Ixioliriaceae NakaiFamily Lanariaceae R. Dahlgren & A.E.van WykFamily Orchidaceae Juss.Family Tecophilaeaceae Leyb.Family Xanthorrhoeaceae Dumort. (including Asphodelaceae Juss. and Hemerocallidaceae R.Br.)Family Xeronemataceae M.W. Chase, Rudall & M.F.Fay
The earlier 2003 version, APG II, allowed 'bracketed' families, i.e. families which could either be segregated from more comprehensive families or could be included in them. These are the families given under "including" in the list above. APG III does not allow bracketed families, requiring the use of the more comprehensive family; otherwise the circumscription of the Asparagales is unchanged. A separate paper accompanying the publication of the 2009 APG III system provided subfamilies to accommodate the families which were discontinued. The first APG system of 1998 contained some extra families, included in square brackets in the list above.
Two older systems which use the order Asparagales are the Dahlgren system and the Kubitzki system. The families included in the circumscriptions of the order in these two systems are shown in the first and second columns of the table below. The equivalent family in the modern APG III system (see below) is shown in the third column. Note that although these systems may use the same name for a family, the genera which it includes may be different, so the equivalence between systems is only approximate in some cases.
The Asparagales include many important crop plants and ornamental plants. Crops include Allium, Asparagus and Vanilla, while ornamentals include irises, hyacinths and orchids.