Pleurothallis, commonly known as bonnet orchids, is a large genus of flowering plants in the family Orchidaceae (order Asparagales). It belongs to the subtribe Pleurothallidinae and is entirely restricted to the New World, making it one of the most species-rich orchid genera in the Americas.
The genus is remarkable for its extraordinary vegetative diversity. Plants may be terrestrial or epiphytic, ranging from tall cane-like stems reaching about a metre in height to trailing, pendent, creeping, tufted, or moss-like miniatures capable of growing on the thinnest of twigs. Despite this variety of form, all species share a single diagnostic feature: two pollinia. Many species have reduced or absent pseudobulbs, with some replacing them with thick, succulent leaves.
The flowers of Pleurothallis are among the most morphologically diverse and unusual in the orchid family, though they are frequently very small. Pollination is typically carried out by tiny insects including gnats, flies, and small wasps.
The center of diversity lies in the high Andes of South America, particularly within the chain of cloud forests running through Colombia. The genus extends broadly across Mexico, Central America, South America, and the West Indies, and tolerates a wide range of climates from dry to wet and from tropical to temperate.
Taxonomically, Pleurothallis was once among the two largest genera in all of Orchidaceae, formerly containing over 1,200 species. Major reclassification work, driven by DNA-based phylogenetic analysis and guided largely by Dr. Carlyle Luer of the Missouri Botanical Garden, led to the creation of numerous segregate genera in 2004—including Acianthera, Brenesia, Crocodeilanthe, and others—reducing the genus by more than half. As of July 2023, Kew's Plants of the World Online recognises approximately 555 species. Allied genera include Dracula, Masdevallia, Restrepia, and Stelis. In horticultural trade the genus is commonly abbreviated as "Pths."
Etymology
The genus name Pleurothallis is derived from the Greek word pleurothallos, meaning "riblike branches," a reference to the distinctly rib-like stems characteristic of many species in the group.
Distribution
Pleurothallis is an entirely New World genus, distributed across Mexico, Central America, South America, and the West Indies. Its greatest concentration of species occurs in the high Andes, particularly in the cloud forests of Colombia. The genus tolerates a wide range of climates, from dry to wet and from tropical to temperate elevations.
Ecology
Species of Pleurothallis occupy terrestrial and epiphytic niches across a broad range of habitats, from tropical lowland forest to cool, moist cloud forest at high Andean elevations. Their tiny, often intricately structured flowers are adapted for pollination by small insects, particularly gnats, flies, and small wasps, with floral diversity likely reflecting tight co-evolutionary relationships with specific pollinators.
Taxonomy Notes
Pleurothallis was historically one of the two largest genera in Orchidaceae, with over 1,200 species, and belongs to the subtribe Pleurothallidinae. DNA-based phylogenetic studies led to a major reclassification in 2004, when numerous segregate genera (Acianthera, Ancipitia, Antilla, Brenesia, Crocodeilanthe, and others) were erected, reducing the genus by more than half. The subgenera Pleurothallis and Specklinia are treated as separate genera by some authorities; subgenus Acuminatae has been proposed as the genus Anathallis. Much of the foundational taxonomic work was carried out by Dr. Carlyle Luer of the Missouri Botanical Garden. As of July 2023, Kew's Plants of the World Online lists approximately 555 accepted species.
