The two largest genera of slipper orchids are Cypripedium and Paphiopedilum. Cypripedium, with about 50 species, is the most widespread genus with a distribution across North and Central America, Europe and temperate Asia (Cribb, 1997). Paphiopedilum is restricted to temperate and tropical Asia and is more speciose, with about 70 species (Cribb, 2011).
The remaining three genera (Phragmipedium, Selenipedium and Mexipedium) are restricted to Central and South America.
In most cases, the five genera of slipper orchids can be distinguished from one another by the combination of locule number in the ovary (uni-locular or tri-locular) and leaf folding: plicate (folded along several veins) or conduplicate (folded lengthwise along one mid-vein, with upper surfaces facing each other).
The exceptions are Paphiopedilum and Mexipedium, which share the same combination of states for these characters, both having a uni-locular ovary and conduplicate leaves. They are separated geographically however; Mexipedium being a monotypic genus endemic to Mexico, while Paphiopedilum occurs only in Asia.
Above: A summary of characters that can be used to distinguish the five slipper orchid genera from one another (diagram by Ruth Bone).
The dotted line separates the three genera with an entirely Central and South American distribution, from the two remaining genera. The two types of leaf folding found in slipper orchids, plicate and conduplicate, are shown in +/- transverse section.
Above: The similarity between Mexipedium and Paphiopedilum (diagram by Ruth Bone).
Paphiopedilum and Mexipedium share the same combination of states for the locule number and leaf folding characters (highlighted in red), both having a uni-locular ovary and conduplicate leaves. They are separated geographically however; Mexipedium being a monotypic genus endemic to Mexico, while Paphiopedilum occurs only in Asia.
Above: A guide to slipper orchid morphology by L. Averyanov.
Illustration by Prof. Leonid Averyanov (Komarov Institute, St Petersburg). Thanks to Prof. Averyanov and Kew Publishing for permission to reproduce this illustration from Slipper Orchids of Vietnam.