A widespread order of lilioid monocotyledons composed of four or five families, the most important of which is Dioscoreaceae (650 species). These families contain some of the most peculiar plants among the monocots. Two families, Thismiaceae and Burmanniaceae (with 35 and 140 species, respectively), are largely composed of nonphotosynthetic herbaceous species that parasitize fungi, whereas other members, such as the bat plant (Tacca, Taccaceae) and the yams (Dioscorea) have net-veined leaves similar to dicots. Most of the 600 species of Dioscorea are vines, and all contain diosgenin, a precursor of progesterone and cortisone, leading to their being commercially collected in some parts of the world.