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Asparagales is an order of plants in modern classification systems such as APG III (which is used throughout this article). 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 were assigned 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 (phytomelanin), 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''). ==Description== 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 in''Dioscorea'' (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''.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Asparagales」の詳細全文を読む スポンサード リンク
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