The Dioscoreales are an order of monocotyledonous flowering plants in modern classification systems, such as the Angiosperm Phylogeny Group and the Angiosperm Phylogeny Web. Within the monocots Dioscoreales are grouped in the lilioid monocots, where they are in a sister group relationship with the Pandanales. The Dioscoreales must contain the family Dioscoreaceae which includes the yam (Dioscorea), some species of which are an important food source in many regions. Older systems tended to place all lilioid monocots with reticulate veined leaves (such as Smilacaceae and Stemonaceae together with Dioscoraceae) in Dioscoreales. As currently circumscribed by phylogenetic analysis using combined morphology and molecular methods, Dioscreales contains many reticulate veined vines in Dioscoraceae, it also includes the myco-heterotrophic Burmanniaceae and the autotrophic Nartheciaceae. The order consists of three families, 22 genera and about 850 species.
Dioscoreales are vines or herbaceous forest floor plants. They may be achlorophyllous or saprophytic. Synapomorphies include tuberous roots, glandular hairs, seed coat characteristics and the presence of calcium oxalate crystals.[5] Other characteristics of the order include the presence of saponin steroids, annular vascular bundles that are found in both the stem and leaf. The leaves are often unsheathed at the base, have a distinctive petiole and reticulate veined lamina. Alternatively they may be small and scale-like with a sheathed base. The flowers are actinomorphic, and may be bisexual or dioecious, while the flowers or inflorescence bear glandular hairs. The perianth may be conspicuous or reduced and the style is short with well developed style branches. The tepals persist in the development of the fruit, which is a dry capsule or berry. In the seed, the endotegmen is tanniferous and the embryo short.[6]
All of the species except the genera placed in Nartheciaceae express simultaneous microsporogenesis. Plants in Nartheciaceae show successive microsporogenesis which is one of the traits indicating that the family is sister to all the other members included in the order.
For the early history from Lindley (1853)[7] onwards, see Caddick et al. (2000) Table 1,[8] Caddick et al. (2002a) Table 1[5] and Table 2 in Bouman (1995).[9] The taxonomic classification of Dioscoreales has been complicated by the presence of a number of morphological features reminiscent of the dicotyledons, leading some authors to place the order as intermediate between the monocotyledons and the dicotyledons.[9]
While Lindley did not use the term "Dioscoreales", he placed the family Dioscoraceae together with four other families in what he referred to as an Alliance (the equivalent of the modern Order) called Dictyogens. He reflected the uncertainty as to the place of this Alliance by placing it as a class of its own between Endogens (monocots) and Exogens (dicots)[10] The botanical authority is given to von Martius (1835) by APG for his description of the family Dioscoreae or Ordo,[3] while other sources[11] cite Hooker (Dioscoreales Hook.f.) for his use of the term "Dioscorales" in 1873[12] with a single family, Dioscoreae.[13] However, in his more definitive work, the Genera plantara (1883), he simply placed Dioscoraceae in the Epigynae "Series".[14]