Anyone who grew up in the northern part of the USA is familiar with lady slipper orchids. As a child I was introduced to this group of plants when I came upon the moccasin flower, Cypripedium acaule, growing in the local woods of southern New York. Little did I know at the time that this species was just one member of a much larger group of plants known as the slipper orchids, subfamily Cypripedioideae. Nor did I realize they are some of the most primitive of the orchid family, so much so that some authorities have considered them a separate family of plants altogether.
Variation within this subfamily is broad in many regards. They can be found growing in a wide range of habitats, from grasslands, to temperate forests, tropical and subtropical forests, and wetlands such as bogs or seepage slopes. Some are completely terrestrial, while others are epiphytic. Both life cycle and habit too are highly varied. Most are evergreen with the exception of the genus Cypripedium which is deciduous, however, as if to prove the variation of these plants, one species, C. subtropicum, is evergreen. The habit of the leaves and the flower scape as well ranges far and wide, though all are sympodial plants, throwing new growths along a trailing rhizome.
So what makes them all slipper orchids? Mostly the characteristics of the flower. The most obvious common feature is the sac shaped lip, giving these plants their common name. Few other orchids have this feature, and none so developed. Moreover, the flower is graced with a staminode - a sterile, shield shaped stamen that covers the two fertile anthers as well as the stigmatic surface. The lip acts as a temporary trap for insects designed such that escape puts them in contact with the staminode, thus increasing the chance of pollination. A few species of Cypripedium are autogamous - self fertilizing.
Another feature common to most species is that the two lower sepals are fused into a single unit called the synsepal. Occasionally, these sepals are not fused, but that is the exception rather than the norm. The remaining sepal is known as the dorsal sepal and often is rather large. The petals are usually held laterally in most species, and in many are the longest flower parts. They frequently hang down, particularly in the genera Phragmipedium and Cypripedium, though certain sections within Paphiopedium have this feature as well.
Five genera are currently recognized. They are as follows:
Cypripedium - this terrestrial genus composed of around 45-47 species is mostly found throughout the northern temperate regions of the world, but outlying species in Central America and southern China fall within subtropical or even tropical climates. They are deciduous in habit and largely woodland plants. Their cultivation is somewhat problematic. As fate would have it, they also are my favorite slipper orchids.
Mexipedium - a single dwarf species is known from Oaxaca, Mexico, M. xerophyticum. This lithophytic plant likes a bit more water in cultivation than its name implies.
Phragmipedium - this tropical American genus numbering around 15-20 species is known for its love of water. Species range from epiphytic, to lithophytic, to terrestrial in habit. It is composed of six sections. The most astounding species to be found recently, P. kovachii, is in a section all its own.
Paphiopedilum - this is by far the most varied group of slipper orchids, containing 80 or so known species, and is found only in southeast Asia and the East Indies. Five subgenera, 15 sections, and 4 subsections can be found within this diverse genus. They range from true epiphytes, to lithophytes, to terrestrial in habit. Most are found within the bounds of truly tropical areas, but some members of the subgenus Parvisepalum venture into distinctly temperate climes. This genus is the most widely grown and hybridized of all slipper orchids.
Selenipedium - an odd group of highly primitive plants, numbering only six species in all, and all confined to northern South America and Panama. Little is known about their culture outside their native lands. Most are quite tall, leafy plants, with relatively small flowers. Curiosities, but never likely to be important in the horticultural trade.