THE orchids are possibly the largest family of flowering plants. Estimates
of the number of species range from 15 000 to 30 000. The imprecision of
the estimates reflects how little we still know of this spectacular family.
One group of orchids that has captured the imaginations of both scientists
and horticulturalists are the slipper orchids. These are a small, isolated
group, distinguished by the slipper-shaped lip of the flowers. A second
characteristic feature is the column, the central part of the flower formed
by the fusion of the reproductive organs. In slipper orchids the column
has two lateral stamens and a large shield-shaped staminode, all derived
from the inner whorl of stamens. In most other orchids, the lip is flat,
with various lobes and carries a large protuberance, the callus, on its
upper surface. The form of the column is also different, with a single anther
derived from the outer whorl of stamens.
Most botanists consider the slipper orchids the most primitive orchids,
partly because their reproductive organs are not so reduced as those of
other orchids and because of their relict distribution in both the Old and
New World. Indeed, some botanists consider that the slipper orchids belong
to a distinct family, the Cypripediaceae. But they have so much in common
with other orchids that most botanists place them in a subfamily, rather
than a separate family.
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Given the enormous number of species of orchid it is perhaps surprising
that there are only about 150 species of slipper orchids. They are widely
distributed, from Europe and Asia to the islands of the southwest Pacific
and throughout the Americas. Most species, however, are strictly endemic,
growing in a very restricted area.
Until the late 19th century, all the slipper orchids were included in
a single genus, Cypripedium. Since then botanists have transferred the 70
or so species from tropical Asia to the genus Paphiopedilum, while the 20
to 25 American species are divided between two genera, Selenipedium and
Phragmipedium. Cypripedium now includes just the 40 or so species from temperate
parts of Europe, America and Asia.
The slipper orchids are vulnerable plants and some are threatened with
extinction in the wild. Their vulnerability stems from three sources. First,
they have exacting ecological requirements. Secondly, their existence depends
on a delicately balanced relationship with a fungal partner. Finally, the
beauty of their flowers has led to commercial exploitation on a large scale.
In nature, slipper orchids are only common in very local areas because
many have such narrow ecological preferences. In Southwest China, for example,
the pink-flowered bubblegum orchid, Paphiopedilum micranthum, grows on isolated
limestone mountains. In Borneo, the spectacular Paphiopedilum sanderianum,
whose petals can reach a metre long, grows only on vertical limestone pinnacles
in one area of Sarawak. Two newly discovered species from Mexico, one a
Cypripedium and the other a Phragmipedium, are so rare that they earned
a place on the country鈥檚 endangered orchids list at the same time as they
were described as new to science.
The biology of slipper orchids makes them susceptible to small changes
in their environment. As with all orchids, the seeds of slipper orchids
are very small because they carry no food store for the embryo. The seeds
germinate in the wild only in the presence of a specific fungus. The thread-like
fungal hyphae penetrate the orchid embryo, which begins to digest them,
so acquiring the nutrients it needs to grow. In many cases this relationship
seems to benefit the orchid even after it has developed green leaves and
can photosynthesise.
Any change to the habitat that disturbs the fungus will also affect
the orchid. This might explain why so many European orchids have dwindled
as land has been drained and fertilised for agriculture.
At the other end of the life cycle, slipper orchids, like any others,
have highly specific means of achieving pollination 鈥� often relying on a
single species of insect. Few species have been investigated but those that
have are intriguing. L. Anders Nilsson, of the Institute of Systematic Botany
in Uppsala, Sweden, studied the pollination of the lady鈥檚 slipper orchid,
Cypripedium calceolus. He found that the flower produces fragrances that
attract solitary bees, mainly females of the species Andrena haemorrhoa.
Red spots on the central staminode mimic the marks, or nectary guides, that
in other flowers direct the bee to nectar. In the lady鈥檚 slipper, the false
trails guide the bee into a position over the mouth of the slipper. The
surface of the staminode is slippery, and the bee falls into the pouch of
the flower. The bee can escape only by crawling up a ladder of hairs that
leads it under the stigma and one of the anthers before emerging at the
base of the flower. When a bee falls for the trick a second time, it deposits
the pollen it collected on the first trip.
Even less is known of the pollination mechanisms of the tropical slipper
orchids. The one documented case is the rare Paphiopedilum rothschildianum.
While studying this species on the slopes of Mount Kinabalu, John Atwood,
of the Marie Selby Botanical Gardens in Florida, discovered that its pollinator
is a hoverfly, Dideopsis aegrota. The fly is attracted to the staminode,
which is covered by many short, white hairs. These apparently resemble the
eggs of aphids, which are the normal food of the fly larvae. The hoverfly
lays its eggs among the hairs. The flies often slip off the staminode and
fall into the pouch of the orchid, which is directly beneath. The pouch
has slippery sides and, as with the lady鈥檚 slipper, the only escape is to
climb the hairy back wall of the pouch to emerge at the base of the column.
The flowers of other species of slipper orchid also seem to be insect
traps. If the pollinator is a rare species of insect then pollination will
not happen often. Being so specialised, and such irregular breeders, makes
the slipper orchids more vulnerable to disturbance of the habitat, and especially
if it affects the pollinator.
The third factor that has brought some species to the verge of extinction
is the fascinating form of the flowers. Slipper orchids are among the most
popular orchids in cultivation. In their case, many collectors seem to prize
true species more than hybrid flowers. The three most commonly cultivated
genera contain so few species that a collector can aspire to grow all of
them.
The current vogue for growing slipper orchids probably began in 1964,
with the discovery of a new species in Thailand 鈥� Paphiopedilum sukhakulii.
Since then, botanists have discovered some 20 new species. Each discovery
has brought a new batch of converts to the cultivation of slipper orchids.
The result has been a tenfold increase in the trade of rare and endangered
species.
Dr Phillip Cribb is the Curator of the Orchid Herbarium at the Royal
Botanic Gardens, Kew.
