Dioscoreaceae

The dioscoreaceae in the strict sense (scientific name Dioscoreaceae sensu stricto, that is, circumscribed excluding the genus relocated to its own family from the taccáceas), are a family of monocotyledonous plants, as circumscribed here they are mostly tropical, more or less herbaceous with small and not very colorful flowers, the ovary is inferior and often with crests or wings on the fruit. Many are climbers recognized by their leaves with a spiral arrangement, with a petiole and a pulvin at the two ends of the petiole, the base of the leaf not surrounding the stem. The lamina has many strongly longitudinal veins and transverse secondary veins, or finely reticulate venation. The family was recognized by modern classification systems such as the 2009 APG III classification system and the APWeb (2001 onwards), although its circumscription varies between the two classification systems, since according to the APWeb it circumscribes it from Strictly, molecular DNA analyzes defined the family as monophyletic if the Taccaceae family (with its only genus Tacca) is excluded from it, which in the APWeb, as of January 2011, constitutes a separate family.

Description

Theoretical Introduction in Descriptive Terminology of Plants

The characters correspond to the circumscription of Judd et al. 2007, and Simpson 2005, both of which include Tacca (in the APWeb in its own family Taccaceae).

Herbs, often climbing vines, perennial, with thick or very swollen tuber-like rhizomes, sometimes with secondary growth, stem with vascular bundles forming 1 or 2 rings in cross section, steroidal and alkaloidal sapogenins commonly present. Hairs simple to stellate, spines ("prickles") sometimes present.

Leaves usually alternate and spiral, opposite or whorled, simple, but sometimes palmately lobed or compound, entire, differentiated in petiole and blade, with palmate venation, the major veins converging and connected by a network of higher order veins, petiole usually with upper and lower pulvin, sometimes with lashes like stipules, not sheathing. Bulbs sometimes present in the axil of the leaves.

Inflorescences determinate, but sometimes appearing indeterminate, axillary, may be a panicle, or raceme, or umbel, or spike of monochasic units (reduced to solitary flowers), in Tacca with prominent involucral bracts.

Flowers usually unisexual (then dioecious plants), or bisexual, radial, pedicelate, bracteate or not, epigynous.

Tepals 6, in two whorls of 3 pieces, all equal to each other, separated to slightly conical, imbricate. Hypanthus absent or present.

Stamens 6 in two whorls of 3 parts, sometimes 3 (only those on the outer whorl), diplostemonous (outer whorl of androecium opposite outer whorl of tepals, and inner one opposite inner whorl of tepals) or antisepals (androecium whorl opposite outer whorl of tepals) filaments separate to slightly conate (then "monadelphus"), free or adnate to base of tepals. The anthers are of longitudinal dehiscence and introrse or extrorse, tetrasporangiate, ditheca.

Pollen monosulcate to variably porate.

Carpels 3, conate, 3 locules, ovary inferior, with axillary or parietal placentation, 1 or 3 terminal styles, 3 stigmas, minute to slightly bilobed. Ovules 1-2 to numerous in each locule.

Nectaries in the septa of the ovaries or at the base of the tepals.

The fruit is usually a triangular, 3-winged loculicidal capsule and 1-3 locular at maturity, but sometimes a berry or samara.

The seeds are usually flattened or winged, the integument with yellow-brown to red pigments and crystals, without albumen. Embryo with 1 or occasionally 2 cotyledons.

See Huber (1998a, b) for a treatment of the family.

Ecology

As circumscribed by Judd et al. 2007 (with Taccaceae), widely distributed in the tropics and subtropics, with a few in temperate regions.

The inconspicuous flowers are pollinated by insects, mainly flies.

Dispersal is usually by wind, as indicated by the specialized fruits: trialate capsules with flattened or winged seeds or samaras (as in species sometimes segregated as the genus Rajania).

Phylogeny

Theoretical Introduction in Philogenia

Dioscoreaceae is placed in Dioscoreales, an order with many vines with reticulate leaves.

They can be distinguished from the phenetically similar Stemonaceae by their trimerous (vs. dimeral) flowers and consistently inferior ovary. They are also easily differentiated from Burmanniaceae, a family of mycoparasitic herbs with scale-like leaves. Smilacacaceae are also vines with reticulate leaves, but can be easily distinguished by their superior ovary, few-seeded berries, and leaves with paired stipular tendrils. Many authors divide Dioscoreaceae sensu lato into several families (for example into Dioscoreaceae sensu stricto and Taccaceae), according to others such tight circumscriptions ignore their shared characters (Caddick et al. 2002a,2002b). However, Stevens at the Angiosperm Phylogeny Website keeps Dioscoreaceae and Taccaceae separate, because they do not appear as sister families in the phylogenetic tree as of the date of publication of this article (January 2011).

The small genus Stenomeris may be sister to the rest, which form a clade supported by putative synapomorphies of subterranean root-tubers, winged or crested fruits, and many-layered seed coat with a crystalline inner layer.

Tacca and Trichopus may have diverged later, and both have perfect flowers.

Tacca is morphologically distinctive and has often been treated in its own family Taccaceae, it is easily distinguished from other members of the family by its acaulescent habit, parietal placentation, and unusual inflorescences Scapose and umbelate with filamentous bracts.

The rest of the species of Dioscoreaceae are dioecious and belong to the large genus Dioscorea. Phylogenetic relationships within this dioecious clade have been investigated on the basis of DNA sequences and morphology (Caddick et al. 2000, 20002a, Wilkin et al. 2005). It is evident that recognition of genera such as Tamus and Rajania make Dioscorea paraphyletic.

Taxonomy

Theoretical Introduction in Taxonomy

The family was recognized by the APG III (2009), the Linear APG III (2009) assigned it the family number 46. The family had already been recognized by the APG II (2003).

The most represented genera are perhaps Dioscorea (400 species) and Stenomeris (200 species) (Judd et al. 2002).

The genera (excluding Taccaceae), together with their valid publication and distribution are listed below (per Royal Botanic Gardens, Kew):

• Godcore.UBE in Philippines Plum. ex L., Sp. Pl.: 1032 (1753). Cosmopolita. (Today includes Tamus and Rajania)
  • Rajania L., Sp. Pl.: 1032 (1753). Caribbean. (Today included in Godcore. so that the latter is not paraphylatic.
• Stenomeris Planch, Ann. Sci. Nat., Bot., III, 18: 319 (1852). West and Malaysia Center.
• Trichopus Gaertn, Fruct. Sem. Pl. 1: 44 (1788). Madagascar, SO from India to Pen. Malaysia.

Judd et al. 2007, Simpson 2005, list Tacca, in their own family Taccaceae on the APWeb.

Genres, according to Watson and Dallwitz 2007:

• Godcore.
• Rajania (Today included in Godcore. so that the latter is not paraphylatic.
• Tamus (Today included in Godcore. so that the latter is not paraphylatic.
• Stenomeris
• Borderea
• Epipetrum

Watson and Dallwitz 2007 place Trichopus in its own family Trichopodaceae.

Economic importance

The "tubers" starchy many species of Dioscorea are edible, these "tubers" they should not be confused with the roots of Ipomoea batatas, which is also known by the same English common name, "yam".

Other species of Dioscorea are valuable medicinally due to the presence of alkaloids or steroidal sapogenins, the latter being used in anti-inflammatory drugs and oral contraceptives. They are also used by indigenous peoples as poisons or soap.