Lilioid monocots

Lilioid monocots (lilioids, liliid monocots, petaloid monocots, petaloid lilioid monocots) is an informal name used for a grade (grouping of taxa with common characteristics) of five monocot orders (Petrosaviales, Dioscoreales, Pandanales, Liliales and Asparagales) in which the majority of species have flowers with relatively large, coloured tepals. This characteristic is similar to that found in lilies ("lily-like"). Petaloid monocots refers to the flowers having tepals which all resemble petals (petaloid). The taxonomic terms Lilianae or Liliiflorae have also been applied to this assemblage at various times. From the early nineteenth century many of the species in this group of plants were put into a very broadly defined family, Liliaceae sensu lato or s.l. (lily family). These classification systems are still found in many books and other sources. Within the monocots the Liliaceae s.l. were distinguished from the Glumaceae.

The development of molecular phylogenetics, cladistic theory and phylogenetic methods in the 1990s resulted in a dismemberment of the Liliaceae and its subsequent redistribution across three lilioid orders (Liliales, Asparagales and Dioscoreales). Subsequent work has shown that two other more recently recognized orders, Petrosaviales and Pandanales also segregate with this group, resulting in the modern concept of five constituent orders within the lilioid monocot assemblage. This has resulted in treating monocots as three informal groups, alismatid, lilioid and commelinid monocots. The lilioids are paraphyletic in the sense that commelinids form a sister group to Asparagales.

The descriptive term "petaloid lilioid monocot" relates to the conspicuous petal-like (petaloid) tepals which superficially resemble true lilies (Lilium). Morphologically, the petaloid or lilioid monocots can be considered to possess five groups (pentacyclic) of three-fold (trimerous) whorls. Lilioid monocots all have flowers which can be considered to have been derived from a lily-like flower with six relatively similar tepals, and six stamens. The typical lilioid gynoecium has three carpels fused into a superior trilocular (three-chambered) superior ovary, axile placentation, a single hollow style, and several ovules with anatropous orientation in one or two rows per locule and nectaries at the base.

However, floral synapomorphy (shared characteristics) is rare since most conform to the general monocot pattern. This pattern is ancestral (plesiomorphic) for the lilioid monocots. Structural monosymmetry is rare, except for Orchidaceae.

Various trends are apparent among the lilioids, notably a change to an inferior ovary and a reduction of the number of stamens to three. In some groups (such as the genus Trillium in the Liliaceae), the tepals have become clearly differentiated so that the flower has three coloured petals and three smaller green sepals. Almost all lilioid monocots retain at least three petal-like tepals. Since some commelinids (e.g., Palisota in the Commelinaceae, Haemodoraceae, Philydraceae, and Pontederiaceae) have petaloid flowers, the term 'lilioid' is a more accurate one for the group which excludes them, since the term petaloid monocot is still occasionally used in describing commelinids. The morphological concept of petaloid monocots has been equated with "animal-attracting" (that is, for pollination) as opposed to wind-pollinating plants (such as grasses) that have evolved very different floral structures. Pollen structure shows that of the two main tapetum types, secretory and plasmodial, the lilioid monocots are nearly all secretory.

In the orders that branched off before the lilioid monocots, the Acorales and Alismatales, flowers differ in several ways. In some cases, like Acorus (Acorales), they have become insignificant. In others, like Butomus (Alismatales), they have six coloured tepals, and so could be called 'petaloid', but stamens and carpels are more numerous than in the lilioid monocots.

The later evolved commelinids have various kinds of flower, few of which are 'lily-like'. In the order Poales, comprising grasses, rushes and sedges, flowers are either petal-less or have small, unshowy petals. Many Zingiberales species have brightly coloured and showy flowers. However, their apparent structure is misleading. For example, the six tepals of cannas are small and hidden under expanded and brightly coloured stamens or staminodes which resemble petals and may be mistaken for them.

In one of the earliest monocot taxonomies, that of John Lindley (1830), the grouping corresponding to the lilioid monocots was the "tribe" Petaloideae. In Lindley's system the monocots consisted of two tribes, the Petaloideae, and the Glumaceae (the grasses and sedges). Lindley divided the Petaloideae into 32 "orders" (roughly corresponding to families) and the Glumaceae into two further orders. Various successive taxonomies of the monocots also emphasized the grouping of species with petaloid (undifferentiated) perianths, such as Bentham and Hooker's Coronarieæ and Hutchinson's Corolliferae ("Corolla bearing") (1936). Hence the concept that there was a natural grouping of monocots whose flowers were predominantly petaloid, gave notion to the term "petaloid monocots". The core group of petaloids were the Liliaceae, hence "lilioid monocots".