The overall morphology of these plants is highly reduced compared to other members of the Ericaceae, which are practically all subshrubs, shrubs, or trees. By contrast, the Monotropoideae are all herbaceaous perennials, in which a seasonal annual shoot reemerges seasonally (in spring or early summer, depending on climate) from a perennial root. The shoot can be characterized as a single inflorescence or cluster of inflorescences, and is generally a raceme with one to many flowers per axis, though occasionally the raceme may be so reduced as to appear similar to a spike, and in Monotropa, the inflorescence can take the form of a solitary flower. Notably, the shoots are achlorophyllous, in keeping with the mycoheterotrophic and non-photosynthetic nature of the plant, and the plants have a striking and distinctive appearance, with coloration ranging from pure white to pastel tones to very bright yellow or red. (If the Pyroleae are included, many of these species are partially photosythentic, and have green vegetative tissue, though leaves are usually reduced to a basal rosette.) The emerging shoots may be erect or nodding, with erect or pendulous flowers, which may become more erect as the plant matures. The flowers themselves, in common with other members of the Ericaceae, have corollas that are generally bell- or cup-shaped, though the petals themselves may or may not be fused. However, the Monotropoideae lack the poricidal anthers that are characteristic of the majority of the Ericaceae. Seeds are highly reduced dust seeds. The shoot may or may not be persistent after seed dispersal.
The monotropes were first described as a distinct plant family, Monotropaceae, by Thomas Nuttall in 1818. David Don was the first to recognize this group as a tribe within the Ericaceae, later raised to subfamily status as the Monotropoideae by Asa Gray in 1878. (However, George Arnott Walker-Arnott was the first to validly publish that name, as a subfamily of Monotropaceae, in 1832, hence, Arnott is cited as author of the name.) Since that time, authors have variously treated this group as a distinct family or as a subfamily of the Ericaceae. Contemporary molecular phylogenetics has clearly established the latter, though many of the details of relationships between the Monotropoideae and the rest of the Ericaceae are still (as of 2014) a topic of active research, particularly the question of whether or not the Pyroleae and the rest of the Monotropeae form a single monophyletic group.
The species in this subfamily are all mycoheterotrophic, relying on fungal hosts for their carbon nutrition. The fungi parasitized by these plants are ectomycorrhizal species of fungi. Hence, these plants act as direct parasites of these fungi, and also indirectly, act as an epiparasite of conifers and the larger shared mycorrhizal network. Monotropoideae species can generally be described as full, obligate mycoheterotrophs, though if the Pyroleae are treated as part of the Monotropoideae, include partially mycoheterotrophic (mixtotrophic) members as well. The parasitism by these plants is generally very specific in terms of its fungal hosts, ranging from single families of fungi, to a few closely-related species. The morphology of the root and the root-level fungal symbiont is distinctive and referred to as monotropoid mycorrhiza, though strictly speaking, it is not a mycorrhiza, which is a mutualistic relationship by definition.
Distribution is through much of the temperate Northern Hemisphere, though ranging into the subarctic and montane tropical regions as well. Distribution is limited by available moisture (Monotropoideae species have limited ability to survive long enough to set seed during seasonal dry periods), and by the distribution of conifer genera that are hosts of the specific host fungi these plants parasitize.
The genera are native to cool temperate and arctic regions of the Northern Hemisphere. The common name wintergreen (shared by several other plants) derives from their evergreen leaves.
The leaves are typically alternate or basal, and are always evergreen. The flowers are regular, most often with five sepals, five petals, and 10 anthers. The fruit are dry dehiscent capsules.
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