Asparagales

Asparagales is an order of flowering plants (angiosperms) within the monocotyledons, comprising approximately 13 families, more than 1,100 genera, and an estimated 36,000 species. It is the largest order of monocots and one of the most species-rich lineages of land plants, owing chiefly to the orchid family (Orchidaceae), the largest family of flowering plants. The order's best-known members include orchids, asparagus, onions and garlic, irises, gladioli, daffodils, agaves, and aloes. Members of Asparagales display extraordinary morphological and ecological diversity, ranging from minute mycoheterotrophic herbs to arborescent yuccas and dragon trees, and the order contains many of humanity's most important food crops, ornamentals, fibers, and medicinal plants.

Description and Morphology

Asparagales is a morphologically heterogeneous order, unified primarily by molecular evidence and a few recurring characters. Most members are perennial herbs, frequently geophytes that survive unfavorable seasons by means of bulbs, corms, rhizomes, or tuberous roots. A substantial minority are woody or arborescent, exemplified by yuccas, dragon trees (Dracaena), agaves, aloes, and the Australian grass trees (Xanthorrhoea); these achieve secondary thickening through anomalous growth rather than true wood.

Leaves are typically parallel-veined with sheathing bases, often arranged in basal rosettes. Flowers are usually bisexual and trimerous, with six tepals in two whorls of three and six stamens; the ovary is superior in some families (e.g., Asparagaceae, Asphodelaceae) and inferior in others (e.g., Amaryllidaceae, Iridaceae, Orchidaceae). Nectaries are commonly situated at the base of the tepals (perigonal nectaries), in contrast to the septal nectaries typical of the related order Liliales. The Orchidaceae deviate markedly from this pattern, possessing one or two functional stamens fused with the style into a column (gynostemium), and pollen aggregated into waxy pollinia.

Several anatomical and reproductive characters have historically defined the order. Pollen tetrads are typically formed by simultaneous microsporogenesis, and the seeds of most families bear a black crust of phytomelan, a carbonaceous deposit in the seed coat; this crust is absent in orchids, whose seeds are minute, dust-like, and lack endosperm. Chemically, the order is characterized by steroidal saponins and calcium oxalate raphides; distinctive alkaloids (such as lycorine and galantamine) occur in Amaryllidaceae, and anthraquinones in Asphodelaceae.

Taxonomic History

The families now placed in Asparagales were historically scattered across the "lilioid" monocots. Following Linnaeus, most were lumped into an enormously broad Liliaceae sensu lato, and twentieth-century systems such as those of Cronquist (1981) and Takhtajan treated most of these groups as part of the order Liliales, while orchids were usually segregated into a separate order, Orchidales.

The modern concept of Asparagales originated with Rolf Dahlgren, who, building on his comparative work and culminating in the 1985 treatment by Dahlgren, Clifford, and Yeo, delimited an order of "asparagoid lilies" on morphological and anatomical grounds, including perigonal nectaries, introrse anthers, simultaneous microsporogenesis, and phytomelan-encrusted seeds. Molecular phylogenetics beginning in the early 1990s confirmed that this assemblage is monophyletic, and the successive systems of the Angiosperm Phylogeny Group (APG, 1998; APG II