* Lohmüller, F.A. 2005. The Botanical System of the Plants [Accessed 24 March 2006]
Asparagales is the name of an order of plants, used in modern classification systems such as the APG system of 2009. The order takes its name from the family Asparagaceae and is placed in the monocots. The order has only recently been recognized in classification systems. It was first put forward by Huber in 1977 and later taken up in the Dahlgren system of 1985. Before this, many of its families belonged to the old order Liliales: a very large order containing almost all monocots with colourful tepals and without starch in their endosperm. DNA sequence analysis indicated that Liliales should be divided into at least Liliales, Asparagales and Dioscoreales. The boundaries of the Asparagales and of its families have undergone a series of changes in recent years; future research may lead to further changes and ultimately greater stability.
The order is clearly circumscribed on the basis of DNA sequence analysis, but is difficult to define morphologically, since its members are structurally diverse. 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), some of which can exceed 10 m in height. Succulent genera occur in several families (e.g. Aloe).
One of the defining characteristics of the order is the presence of phytomelan, a black pigment present in the seed coat, creating a dark crust. Phytomelan is found in most families of the Asparagales (although not in Orchidaceae, thought to be a sister to the rest of the group).
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; the leaves are less often 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.
The order is thought to have first diverged from other related monocots some 120-130 million years ago (early in the Cretaceous period), although given the difficulty in classifying the families involved, estimates are likely to be uncertain.
From an economic point of view, the order Asparagales is second in importance within the monocots to the order Poales (which includes grasses and cereals). Species are used as food and flavourings (e.g. onion, garlic, leek, asparagus, vanilla), as cut flowers (e.g. freesia, gladiolus, iris, orchids), and as garden ornamentals (e.g. day lilies, lily of the valley, Agapanthus).
Most species of Asparagales are herbaceous perennials, although some are climbers (e.g. species of Asparagus, family Asparagaceae) and some are tree-like. The order also contains many geophytes (bulbs, corms and various kinds of tuber). Almost all species have a tight cluster of leaves (a rosette) at the base of the plant or, in the tree-forming species, at the end of a woody stem. Only in a few cases are leaves produced along the length of the stem. The flowers are often at the tip of the stem and are mainly of a rather generalized 'lily type', with six tepals and up to six stamens.
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.
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 families of the Asparagales, as set out in the APG III system, are briefly surveyed below. For more information follow the links to the main articles.
The orchid family is one of the two largest families of angiosperms (the other is Asteraceae). The shape of the flowers is very distinctive, making orchids easy to recognize. The flower is bilaterally symmetrical. The three sepals are generally colourful and bright (which is why they are sometimes called outer tepals), with one on each side ("lateral sepals") and one usually at the top of the flower ("dorsal sepal"), sometimes forming a hood. The three petals (or inner tepals), also showy, are located alternately between the sepals, two at the side and one usually at the bottom of the flower. The lower petal is referred to as the "labellum" or "lip", and is usually distinctively different from the side petals. Thelymitra is an example of a genus where the lower petal is similar in appearance to the other petals. The pollination systems of orchids are among the most complex and interesting of all the angiosperms.
Orchids include many species of great ornamental value. Vanilla is obtained from the fruit of the orchid Vanilla planifolia.
The genus Borya contains tree-like species which behave as "resurrection plants". Growing on rocky slopes, the plants dry out during the dry season and become a rusty orange color, but quickly turn green and become active again once it starts to rain. Along with the other genus in the Boryaceae family, Alania, these xerophytic plants are native to Australia.
Blandfordia is the only genus in the family Blandfordiaceae, with four species distributed in eastern Australia. They are commonly called "Christmas Bells", because of the shape of their flowers and their flowering time, which coincides with Christmas in Australia. They are upright perennial herbs (to about 1.50 m), with distinctive leaves. The inflorescence is a raceme. Individual flowers have jointed pedicels and tepals forming a tubular shape. The seeds have conspicuous hairs.
Lanaria lanata is the only species in the family Lanariaceae and is found in southern South Africa. A more or less typical monocotyledon, the species can be recognized by its shortly branched inflorescence covered with branched hairs (giving rise to the common name of Lamb's-tail). The flowers are radially symmetrical.
The Asteliaceae is a family of two to four genera of plants found in the Southern Hemisphere. They are more or less rhizomatous, with spiral leaves and an inflorescence that may form a raceme or a spike. There are large bracts at the base of the inflorescence. The individual flowers are small, with tepals joined at the base.
The family includes some 150 species with a worldwide distribution, excluding Europe and northern Asia. Species can be recognized by their rosettes of more or less folded leaves with persistent bases and quite prominent nonglandular hairs. The tepals in the outer whorl tend to be green on the outside. The ovary is inferior with often a thin tubular portion at its apex formed by joined tepals or the tip of the ovary.
The family includes a single genus, Ixiolirion, with four species distributed from Egypt to central Asia. They are herbs with corms and an inflorescence forming a cluster. The individual flowers are blue, shortly tubular, with an inferior ovary.
The nine genera are found in Chile, the United States and Africa. They are herbs with corms and leaves which are sometimes stalked (petiolate) with wide blades. The flowers have tepals that open outwards. The stamens are strongly dimorphic. The anthers open by pores.
The genus Cyanastrum is sometimes placed in its own family Cyanastraceae.
The two species of Doryanthes, the only genus of the family, are huge rosette-forming herbs that are a conspicuous element of the flora in the vicinity of Sydney, Australia, being hard not to notice when in flower. The leaves have entire margins but disintegrate into fibres at the apex. The sub-umbellate inflorescences are borne at the end of long stems, having numerous bright red flowers, which are radially symmetric with inferior ovaries.
The iris family contains about 70 genera and over 1,600 species with a worldwide distribution.
Members of the family are usually perennial herbs with sword-shaped unifacial leaves; the inflorescence is a spike or panicle of solitary flowers, or forms a monochasial cyme or rhipidium (meaning that the successive stems of the flowers follow a zig-zag path in the same plane); and the flower has only three stamens, each opposite to an outer tepal.
Saffron is obtained from the dried styles of Crocus sativus L., a member of the iris family. The corms of some species of Iridaceae are used as food by some indigenous peoples.
Many species in the iris family have a great economic importance in ornamental horticulture and the cut flower industry, especially Gladiolus, Freesia, Sparaxis, Iris, Tigridia (tiger lily), Ixia (corn lily), Romulea, Neomarica, Moraea (butterfly lily), Nemastylis, Belamcanda, Sisyrinchium (blue-eyed grass), Crocosmia and Trimezia. Many other genera, both perennials and bulbs, are grown in gardens in tropical and temperate regions (e.g. Watsonia, Crocus, Dietes, Tritonia, Hesperantha and Neomarica).
Moraea and Homeria are two genera of poisonous plants which are a problem in sheep and cattle producing regions, notably in South Africa.
The family consists of a single genus Xeronema with two species, one found only on the Poor Knights islands in New Zealand and the other in New Caledonia. The inflorescence is crowded with quite large, radial symmetrical flowers, which face upwards. The stamens are strongly exserted (i.e. extend out of the flower). The family is still poorly known.
Xanthorrhoeaceae sensu lato
The Xanthorrhoeaceae, or grasstree, family has been recognized by most taxonomists, but the limits of the family have varied greatly. In the narrowest definition, the Xanthorrhoeaceae sensu stricto consists only of the genus Xanthorrhoea. Based on phylogenetic research, the latest (2009) revision of the APG classification groups together the former families Hemerocallidaceae, Xanthorrhoeaceae sensu stricto and Asphodelaceae as the Xanthorrhoeaceae. A paper published at the same time proposed that the original three families should be retained as subfamilies within Xanthorrhoeaceae sensu lato. This division has been used here.
The Hemerocallidoideae, or day lily, subfamily of the Xanthorrhoeaceae sensu lato is treated in some systems as a separate family, the Hemerocallidaceae.
It includes perennial herbaceous plants which are glabrous and have short rhizomes with fibrous roots or are rhizomatous with root tubers. The leaves form a rosette at the base of the plant, and are alternate, distichous, flat, sessile, simple, linear or lanceolate, and parallel veined, with entire margins. The flower is typically somewhat zygomorphic (i.e. not radially symmetrical) and has nectaries. The flowers are arranged in various types ofinflorescence. The group includes eight genera and about 85 species distributed in the temperate zones of Europe and Asia, Malaysia, India, Madagascar, Africa and the Pacific, from Australia and New Zealand to South America.
Two of the genera, Hemerocallis (day lily) and Phormium (New Zealand flax), are grown as ornamentals worldwide.
The Xanthorrhoeoideae, or grasstree, subfamily of the Xanthorrhoeaceae sensu lato is treated in some systems as a separate family, the Xanthorrhoeaceae sensu stricto.
It contains only one genus, Xanthorrhoea, endemic to Australia. Many species have an erect woody stem which is covered with persistent dried leaves unless there have been fires, topped by a crown of long thin leaves. The spike-like inflorescence is erect and densely crowded with small flowers. The fruit is a capsule. Plants are adapted to bush fires, which can stimulate flowering.
The Asphodeloideae, or asphodel, subfamily of the Xanthorrhoeaceae sensu lato is treated in some systems as a separate family, the Asphodelaceae.
Members of the family are natives of temperate to tropical regions of the Old World, with 15 genera and 780 species. The greatest diversity occurs in South Africa, usually in arid habitats. They differ from other related families by often being pachycauline (i.e. with a thickened trunk, usually wider at the base, which has a water storage function), by usually having succulent leaves, and by possessing a trimerous flower with a superior ovary and seeds with an aryl.
The most conspicuous genus in the family is Aloe. Many species of Aloe are used medicinally and in cosmetics. For example, "aloin" is derived from Aloe vera and Aloe ferox and has important medical uses (e.g. as a laxative and in the treatment of burns) as well as cosmetic uses (e.g. in skin and hair products). Other genera are used as ornamental plants, both succulents such as Aloe, Haworthia and Gasteria and perennials such as Kniphofia, Asphodelus and Bulbine.
Amaryllidaceae sensu lato
The amaryllis family has been recognized in many taxonomic systems, but the limits of the family have varied. In the narrowest definition, the Amaryllidaceae sensu stricto is characterized by an umbellate inflorescence with an inferior ovary. Two other groups have similar inflorescences but a superior ovary, and have at times been put into separate families: the Agapanthaceae and the Alliaceae. Based on phylogenetic research, the latest (2009) revision of the APG classification groups together these three families under the conserved name of Amaryllidaceae. (Earlier the APG had used the name Alliaceae for this group.). A paper published at the same time as the 2009 classification proposed that the original three families should be retained as subfamilies within Amaryllidaceae sensu lato. This division has been used here.
The agapanthus subfamily of the Amaryllidaceae sensu lato is treated in some systems as a separate family, the Agapanthaceae.
Agapanthus, native to South Africa, is the sole genus of the subfamily. They are relatively robust herbaceous perennials with short rhizomes and leaves forming a rosette, individually linear-oblong, flat, rather fleshy. The flowers are quite large, blue or white, forming an umbel at the end of a stem (scape) which is longer than the leaves. The inflorescences are protected by bracts joined together along one side. The ovary is superior. Plants do not have the characteristic garlic odor of the allium subfamily (Allioideae). They are set apart from the amaryllis subfamily (Amaryllidoideae) by their superior ovary, the presence of saponins and the absence of the alkaloids typical of amaryllids.
Agapanthus is widely grown as an ornamental in temperate gardens.
The allium subfamily of the Amaryllidaceae sensu lato is treated in some systems as a separate family, the Alliaceae.
Members of the subfamily are found worldwide, in temperate, subtropical and tropical regions. They are herbaceous perennials, usually with bulbs, although in some cases they have short rhizomes. The subfamily can be easily recognized by its characteristic smell (the smell of garlic and onions, singular enough to be called "garlic odour"), by the very soft, fleshy leaves and the umbel-like inflorescence at the end of a stem (scape), which has small to medium flowers with a superior ovary.
The subfamily is of considerable economic importance, being grown as vegetables and seasoning, medicinal plants and ornamentals. The genus Allium includes some of the most widely used edible plants, such as onion and shallot (varieties of Allium cepa), garlic (A. sativum and A. scordoprasum), leek (Allium ampeloprasum var. porrum), and various flavourings such as chives (Allium schoenoprasum).
The organosulphur compounds responsible for the characteristic odour are believed to have antioxidant, antibiotic and anticarcinogenic properties, to stimulate the immune system and to be protective of liver functioning.
The family also has important ornamentals, mainly from the dominant genus Allium, but also including genera such as Ipheion.
The amaryllis subfamily of the Amaryllidaceae sensu lato is treated in some systems as a separate family, the Amaryllidaceae sensu stricto.
The subfamily includes 59 genera and approximately 800 species from temperate and tropical regions worldwide. They are herbaceous perennials with bulbs, and can be identified by their rather fleshy leaves, usually large and attractive flowers, with six stamens and an inferior ovary. The flowers are solitary or, more frequently, arranged in umbellate inflorescences at the end of a stem (scape).
Many species of Amaryllidoideae are popular as ornamentals in parks and gardens. A special mention should be made of Narcissus (daffodils and narcissi), cultivated in various parts of the world as an ornamental in gardens and as a cut flower.
Asparagaceae sensu lato
The members of this group have a complex taxonomic history, having been assigned to widely differing families in different classification systems. Proposed subgroups are difficult to recognise, having similar 'lily-like' flowers, with the result that some members of the group have been included in different subgroups at different times. Based on phylogenetic research, the latest (2009) revision of the APG classification supports the use of a single broadly defined family, Asparagaceae sensu lato. A paper published at the same time as the 2009 classification proposed seven subfamilies for the families recognized in the very first APG classification of 1998. This division has been used here, although it is not clear whether the approach will be upheld by future research as some of the clades are weakly supported.
The broadly defined family is large, with some 153 genera and 2480 species, and occurs worldwide.
The Aphyllanthoideae subfamily of the Asparagaceae sensu lato is treated in some systems as a separate family, the Aphyllanthaceae.
It comprises a single species, Aphyllanthes monspeliensis, found in arid areas of the western Mediterranean. The inflorescence is made up of small clusters of blue flowers at the end of a long stem (scape). An unusual feature of the species is that the stem (scape) is actually the main photosynthetic organ, since the paper-like leaves at the base lack chlorophyll.
The Brodiaeoideae subfamily of the Asparagaceae sensu lato is treated in some systems as a separate family, under the name Themidaceae.
It comprises about a dozen genera which are native to western North America. Plants are superficially similar to those of the allium subfamily, being perennial herbs with an umbellate inflorescence made up of quite small flowers. The tepals are more or less joined at the base, sometimes with a corona (a structure like the trumpet of a daffodil). The ovary is superior. Plants lack the "garlic odor" typical of the allium subfamily, and have a fibrous corm rather than a bulb. The inflorescence bracts also differ from those of alliums.
A number of genera, including Brodiaea and Triteleia, are grown as ornamental plants.
The Scilloideae, or scilla, subfamily of the Asparagaceae sensu lato is treated in some systems as a separate family, under the name Hyacinthaceae.
The group includes from 770 to 1,000 species, distributed predominantly in Mediterranean climates, especially South Africa, the Mediterranean to Central Asia and Burma, and South America. Characteristics of the subfamily include: flowers with six tepals and six stamens, typically arranged in a raceme; a superior ovary; growing from bulbs; rather fleshy mucilaginous leaves in a basal rosette. Plants contain poisonous compounds, so that they are not edible.
Many spring- and summer-flowering bulbs grown in gardens in temperate climates belong to this subfamily, including genera such as Scilla (squill), Muscari (grape hyacinth), Hyacinthus (hyacinths),Chionodoxa (glory of the snow) and Galtonia (summer hyacinth). Some are used as cut flowers.
The Agavoideae, or agave, subfamily of the Asparagaceae sensu lato is treated in some systems as a separate family, under a variety of names, including Agavaceae. It includes species formerly placed in several other families (e.g. Anthericaceae and Hesperocallidaceae). Many species currently assigned to this group have been placed in other groups at different times. Stevens notes "The broad concept of Agavoideae adopted here may not seem very satisfactory, but I fear that none of the alternative solutions is much better ...".
Given this broad definition, there are about 23 genera in over 600 species, distributed more or less around the world outside cold areas. South west North America, including Mexico, is an area of particular diversity. Some members of the subfamily form trees (such as the Joshua Tree, a species of Yucca). They often have large, succulent leaves in rosettes, either at the base or at the end of the branches. Others are herbaceous (e.g. Hosta, Anthericum). The flowers have six tepals and six stamens with either a superior or inferior ovary.
Agave has important economic uses (for example, it is used to make tequila and mezcal). Some genera are used as ornamental garden plants in temperate regions (e.g. Hosta) and as house plants (e.g. Sansevieria, Chlorophytum).
The Lomandroideae subfamily of the Asparagaceae sensu lato is treated in some systems as a separate family, Laxmanniaceae.
The group consists of some 15 genera and about 180 species from Australasia, south east Asia, and South America. The best known genus is Cordyline. The tepals of the flower persist in the fruit.
Species of Cordyline are grown as house plants and as garden plants in temperate to tropical regions.
The asparagus subfamily of the Amaryllidaceae sensu lato is treated in some systems as a separate family, the Asparagaceae sensu stricto.
The subfamily contains only two genera, one, Asparagus, with around 150-300 species distributed throughout the Old World and a small area of Australia, the other, Hemiphylacus, with only five species, found in Mexico. The photosynthetic organs of Asparagus have been the subject of some controversy; however, most authors consider them to be flattened stems rather than leaves (phylloclades). The leaves are reduced to non-photosynthetic scales, with the phylloclades in their axils. The flowers are small, bell-shaped, greenish-white to yellowish, with six tepals partially joined together at the base, either single or in small clusters, springing from the junctions of the phylloclades. Asparagus species are usually dioecious, with male and female flowers on separate plants. The fruit is a small red berry, which is poisonous to humans.
Asparagus officinalis is used as a vegetable, the young shoots being cut before they become woody. Other species are used as house plants and as greenery in the cut flower trade.
The Nolinoideae subfamily of the Asparagaceae sensu lato is treated in some systems as a separate family, under a variety of names, including Ruscaceae sensu lato. As with the subfamily Agavoideae, the Nolinoideae contains genera previously classified in a number of different families (including Ruscaceae sensu stricto, Nolinaceae sensu stricto, Convallariaceae sensu stricto and Eriospermaceae sensu stricto).
When broadly defined, the group contains some 26 genera and almost 500 species, distributed mainly in the temperate to tropical regions of the Northern Hemisphere. There are few morphological features separating the subfamily from other groups within the Asparagaceae sensu lato. The small flowers are radially symmetrical, with six tepals, usually joined at the base, six stamens and a superior ovary. The fruit is usually a berry with few seeds. Species vary from herbaceous perennials to tree-like forms (e.g. Dracaena).
There are several examples of convergent evolution between species in this subfamily and those in other subfamilies of the Asparagaceae sensu lato. Ruscus (butcher's broom) has photosynthetic branches (phylloclades), similar to those of Asparagus (Asparagoideae); Dracaena draco has a tree-like habit resembling Yucca (Agavoideae) and Cordyline (Lomandroideae).
Some genera are used in horticulture: Sanseviera and Aspidistra are used as house plants in temperate areas and as garden plants in warmer regions; Polygonatum and Ophiopogon are used as garden plants in temperate areas.
The order Asparagales has only recently been recognized in classification systems, with the advent of phylogenetics. Older systems based on morphology placed many of the species now included in Asparagales into the lily family (Liliaceae); where other families were recognized, these were generally placed into the old order Liliales: a very large order which was used for 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. Thus the Wettstein system, last revised in 1935, did not recognise the Asparagales, and placed many of the plants involved in an order called 'Liliiflorae'. The widely used Cronquist system of 1968-1988 also did not recognise Asparagales, using instead the very broadly defined order Liliales.
The 2009 revision of the Angiosperm Phylogeny Group system, APG III, places the order in the clade monocots with the circumscription given below. The APG III system is very recent, as of December 2010[update], so it is not yet employed in many text books; however it is likely to become more influential since its family circumscriptions are being used as the basis of the Kew World Checklist of Selected Plant Families. With this circumscription, the order consists of 14 families with approximately 1120 genera and 26000 species.
Order Asparagales Link
* Family Amaryllidaceae J.St.-Hil. (including Agapanthaceae F.Voigt, Alliaceae Borkh.)
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 other 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 APG III system 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.
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 of the commelinid clade.
A possible phylogenetic tree for the Asparagales, including those families recently reduced to subfamilies, is shown below.
The tree shown above can be divided into a basal paraphyletic group, the 'lower Asparagales', from Orchidaceae to Xanthorrhoeaceae sensu lato, and a well-supported monophyletic group of 'core Asparagales', comprising 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.
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 Cypripedioideaehave 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.
Boryaceae to Hypoxidaceae
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.
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[update], 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.
Notes and references
1. ^ a b c d e Angiosperm Phylogeny Group III 2009
* Angiosperm Phylogeny Group II (2003), "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II", Botanical Journal of the Linnean Society 141: 399–436, http://w3.ufsm.br/herb/An%20update%20of%20the%20Angiosperm%20Phylogeny%20Group.pdf
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