1995 - Taxonomic classification
PHRAGMIPEDIUM XEROPHYTICUM AND ITS CULTURE
Koopowitz, H.
Orchid Digest 59(3): 108-110. 1995
A NEW AND UNUSUAL SPECIES of slipper orchid was described from Mexico in 1990. This is a plant adapted to dry environments, possessing thick and succulent leaves. The flowers are very tiny and whitish in color. The species is distinctive from other known slipper orchids and was described as Phragmipedium xerophyticum (Soto, Salazar & Hagsater, 1990) because of its apparent habitat preference. In some respects, this new species seems to resemble two other Phragmipedium species, P. schlimii and P. besseae, but the flowers have rather narrow incurved petals and are much smaller than even the tiny P. schlimii. Barely one centimeter in natural spread, the P. xerophyticum blossoms are white with a slight pinkish flush. Like P. besseae, P. xerophyticum is able to spread by means of runners . The plants, however, are small with very thick succulent leaves, barely reaching 5 cm in length.
Only seven plants were originally found and pieces of them were brought into cultivation. The exact locality was not disclosed as the discoverers were scared that the plants might be collected out by unscrupulous traders. There was good reason for this concern because the unique P. exstaminodium was exterminated by illegal collection in Mexico within a few years of its discovery. A piece ofP. xerophyticum was sent to a grower in the United States who was successful in propagating it and has since been able to distribute pieces to other growers and institutions. Because the collected plants do not appear to have grown well in Mexico, the plants in the United States have become very important and are now being used to mass produce this interesting species. At least two commercial nurseries have been successful in producing embryos and seedlings from selfed pods and already flasked seedlings have been offered in the trade. This demonstrates the crucial role that commercial enterprises can play in the conservation of rare orchid species. Due to their efforts, this very rare species will be spread in cultivation even though its continued existence in the wild is uncertain. lt might be pointed out that having been transferred to the new genus Mexipedipedium that the species may have lost the CITES protection afforded it as a Phragmipedium.
For those growers who manage to obtain seedlings or divisions of P. xerophyticum, some notes on its cultivation may be useful. Remember these are succulent plants that normally grow in a dry environment. The cultural requirements have been worked out by Tom Kalina at Fox Valley Orchids who has succeeded with the species. Plants are exposed to brighter light and somewhat drier conditions than usually used for other Phragmipedium species. They appear to grow well in the following mix of six parts fine bark to one part #3 charcoal and one part of coarse perlite. In this mix all the particles are approximately the same size which allows for very good drainage.
Two years after being described, P. xerophyticum was placed into a genus of its own, Mexipedium (Alberts and Chase, 1992) . This move has been controversial. Alberts and Chase based their decision on three characteristics wherein P. xerophyticum differed from other Phragmipedium species. One major factor that led to the separation was the observation that anatomically, the ovary has a single cavity like Paphiopedilum, instead of the three separate cavities as in Phragmipedium. The other two features were not spelled out but are probably biochemical differences in the sequence of nucleotides making up a protein gene known as rbcL. A morphological difference should probably be weighted more heavily than a simple chemical substitution difference, because it is highly unlikely that profound changes in anatomical organization are the result of a single chemical substitution within a gene. lt is also possible that the chemical differences could occur in other members of the genus which have not yet been tested.
Such a significant morphological difference therefore was seen by Alberts and Chase as being of major importance. The gen era of the slipper orchids vary as follows. Both Cypripedium and Paphiopedilum have ovaries with a single central cavity whereas Phragmipedium and Selenipedium have three cavities in their ovaries. A three-cavity ovary is considered to be a primitive characteristic (Dressler, 1993 ) and is not widespread in the Orchidaceae. The genera Neuwiedia and Apostasia which are often included in the Orchidaceae as its most primitive members have three-chambered ovaries. Other primitive orchids such as Vanilla share the same characteristic.
Two years after being described, P. xerophyticum was placed into a genus of its own, Mexipedium (Alberts and Chase, 1992).
One would expect that ovaries with a single chamber evolved from three-chambered forms by the loss of the central dividing wall. That would make Selenipedium and Phragmipedium the most primitive groups from which Cypripedium and Paphiopedilum later evolved. Otherwise, there could have been a common ancestral form with a three-chambered ovary that gave rise to each of the present groups. Various orchid groups then evolved single-chambered ovaries and probably did so independently of each other.
One might suppose that the single-ovary chamber in P. xerophyticum is simply an adaptation brought about by the small size of the flower whose ovary is only 2 mm in diameter. Perhaps in this case the reduction in ovary-locule number is merely a simplification needed to accommodate the size reduction. Similar simplifications are found in all organisms which are reduced in size and these usually have little evolutionary significance. This plant can be compared to other Phragmipedium species which also have tiny flowers. A good example would be the very reduced flower size in forms of P. pearcei var. ecuadorense, in particular those forms that used to be known as P. ecuadorense var. gracile.
In a remarkably short time Carol Woodin has established herself as one of the top botanical illustrators of orchid plants in the United States. Carol is a self-taught botanical artist. While painting woodland wildflowers in 1990, she stumbled upon a group of Cypripedium acaule and, since that serendipitous discovery, has painted only orchids. Some of her work was displayed in Boston at the American Orchid Society Trustees' meetings in October 1992 and she also showed her work at the 14th World Orchid Congress in Glasgow in 1993 where she was awarded a Silver Medal. Her display of orchid art received a first prize at the 1993 Greater New York Show at the World Trade Center in April of 1993. She was commissioned to paint Phragmipedium besseae for the 1993 EOC poster. She is currently working on the illustrations for the upcoming book The Genus Phragmipedium by Phillip Cribb.
Teratology is the term used to describe effects where accidents during development cause the formation of unusual characteristics. Sometimes these aberrations are genetically fixed. The genus Phragmipedium has several other well-known teratological "species." These include P. exstaminodium which always has an extra stamen and P. caudatum var. lindenii where the pouch always takes on the characteristics of a third petal. The single-cavity ovary of P. xerophyticum easily falls within this type of aberration.
There are only a few other anatomical features that consistently separate Phragmipedium from Paphiopedilum. These special characteristics of Phragmipedium include: the presence of branched inflorescences, valvate aestivation of the buds (i.e ., the sepals do not overlap in the bud stage as occurs in Paphiopedilum), and the presence of an anther-depressor mechanism. The anthers in Phragmipedium have evolved a ledge or protrusion associated with the pollen masses. As the pollinator brushes past the anthers it pushes against this depressor which then causes the anther to swivel and presses the pollen mass against the pollinator's body. For some reason this unique adaptation of Phragmipedium seems to have been overlooked by nearly all other workers on the group. These three unique characteristics help to define Phragmipedium as a natural clade that shares a common ancestry. The new Mexican species possesses all three characteristics. The later acquisition of a single locule hardly seems enough grounds to erect a new genus. As Cribb has pointed out (1982), other features of the flower, such as the inrolled distal portion of the pouch, have appeared independently within sections of Phragmipedium, e. g., in Phrag. besseae and in Paphiopedilum in both the subgenera Brachypetalum and Parvisepalum, as well as in the genus Cypripedium. Therefore there is little reason to separate P. xerophyticum from the rest of the genus because it has one morphological feature that is unusual.
The various genera of the slipper orchids appear to be reproductively isolated from one another. Despite the several intergeneric registrations by orchid enthusiasts and the one American Orchid Society quality award to a putative Phragmipaphium hybrid, there are no conclusively demonstrated hybrids between Phragmipedium and Paphiopedilum. I was present at the judging of Phragmipaphium Hanes Magie and was hardpressed to find any features that were conclusively Phragmipedium in either the plant or the flower. On the other hand, within each genus hybrids can be easily made that are intergrades between any combination of species. The breeding behavior of P. xerophyticum sheds further light on this plant's affinities. Phragmipedium xerophyticum does appear to cross readily with other members of the genus Phragmipedium and hybridizers have produced seedlings using that species. This is an additional suggestion of where it really belongs.
Whether or not the new generic name will be accepted remains to be seen. Generic names are really artificial contrivances used for the convenience of man as a way of delineating groups of species. Ideally they also unite species that share a common evolutionary origin. But generic limits can be set at several different levels and to some it hardly seems worthwhile to erect a new genus for this Mexican species that so clearly is allied to the other Phragmipediums.