1996 - Botanical description
MEXIPEDIUM XEROPHYTICUM
ORCHIDACEAE
Mark W. Chase
Curtis’s Botanical Magazine
Volume 13 Part 3 August 1996
Summary
The history, taxonomy, distribution, ecology, conservation status and cultivation of the Mexican endemic Mexipedium xerophyticum (Orchidaceae) are discussed; a full botanical description is provided.
When it was first described by Soto Arenas, Salazar & Hagsater (1990), this species was placed in the genus Phragmipedium, but the decision was somewhat controversial at that time, even by the authors' own admission. First of all, it exhibited some traits more typical of the Asian genus Paphiopedilum, such as a parietal placentation rather than the axile condition otherwise found in Phragmipedium. Moreover, it really did not compare closely with any species in either genus. The authors chose wisely not to take a controversial decision at a time when no overall phylogenetic studies of the slipper orchids had been conducted. Knowing how it fitted into the general pattern of relationships was critical to determining the most appropriate taxonomic treatment. Many taxonomic decisions are made without this sort of perspective, and are then amended by the next worker, whose subjective considerations result in a revision of the taxonomy, and so on. Such changes should be adopted only when new evidence is available and interpreted phylogenetically. This alone will result in a stable taxonomy that can then stand scrutiny. In the case of M. xerophyticum, the decision to place it in a separate genus was based on both morphological and chloroplast DNA studies (Albert & Chase, 1992).
The problem posed by M. xerophyticum was that although its closest relationships were demonstrated to be with Phragmipedium, it could not be united with Phragmipedium without losing the distinctive combination of characters that separated Phragmipedium and Paphiopedilum. There were two choices available at that time: to place it in its own genus or to expand Paphiopedilum. The first option was favoured but, subsequent to the description of Mexipedium, Albert & Pettersson (1994) argued that the overall pattern of relationships would be best served by submerging Mexipedium and Phragmipedium into Paphiopedilum. This expansion of Paphiopedilum has not been widely accepted. Either treatment would be acceptable by the tenets of phylogenetic classification.
Regardless of its taxonomic treatment (I use here Mexipedium), this species is a most peculiar and intriguing plant. lts native habitat in the state of Oaxaca, Mexico, is unusual for a slipper orchid: a seasonally dry, relatively open scrub dominated by cacti (Acanthocereus), century plants and other succulents (Agave, Nolina, and Yucca), and FRANGIPANI (Plumeria). The species epithet refers of course to its thick and presumably drought-adapted leaves, and this part of Mexico does experience a prolonged dry period in the winter. The plant is minute and inconspicuous; it is definitely not among the showiest of the slipper orchids. There is little apparent justification for its use in hybridization, even though crosses with Phragmipedium have been reported (H. Koopowitz, pers. comm.). The fact that it does hybridize with Phragmipedium should not be considered significant with respect to classification because many closely related orchid species do not cross whereas some distant relatives do.
Ecology and cultivation. Few plants of his narrow endemic are in cultivation, and thus relatively little is known about their cultural requirements. Presumably, they should be grown in an open, well-draining medium with a complete cessation of winter watering, which corresponds to the dry period in Mexico. During summer, abundant water is available in its natural habitat. lt is reported to grow in sites with no direct sun during the middle of the day and often in pockets of humus with Selaginella and terrestrial bromeliads on rocks.
Conservation status. In Mexico, Mexipedium is considered in danger of extinction; at the time of its description only seven clones were known. The original locality has never been published and is not included on the herbarium specimens so as to protect it from collectors, who would undoubtedly endanger this rare plant if they knew the exact locality. Plants should not be purchased since it is unlikely that these will have been grown from seed. Like most slipper orchids it is included in Appendix I of CITES and is thus prohibited from being in international commerce.
It is impossible to confuse this species with any other slipper orchid. The combination of its thick, succulent leaves, small stature, and whitish flowers on a weakly branching inflorescence is unique.
Description. Terrestrial or lithophytic herb. Leaves 3–7, conduplicate, linear-oblong, fleshy-coriaceous, green, the largest being 3.5–12 cm long, 1.2–1.8 cm wide, approximately 1 mm thick, with an unequal mucronate tip and carinate below. lnflorescenceweakly paniculate, producing one flower at a time over a long period; peduncle 6.5–13.5 cm long, slender, reddish, shortly red-pubescent; bracts elliptic, obtuse, 8–15 mm long, 4–9 mm wide, dark brown, pubescent. Flowers 1.5–2.0 cm in diameter; dorsal sepal white with reddish pink reverse; synsepal similar; petals white; lip white; staminode white with a rosy-pink mid-lobe. Pedicel and ovary red-green pubescent, 2.5–3.0 min long, 0.8 mm in diameter in the middle. Dorsal sepal elliptic, 0.9–1.4 cm long, 0.5–0.7 mm wide, pubescent on the outer surface. Synsepal suborbicular, obtuse, 0.8–1.0 cm long, 0.8–1.2 cm wide, pubescent on the outer surface. Petals incurved, nearly horizonal, linear-ligulate, 1.1–1.5 cm long, 0.2–0.3 cm wide, obtuse, nearly glabrous or sparsely pubescent at base, margins undulate. Lip subglobose, inflated, 1.0–1.4 cm long, 0.7–0.9 cm wide. Staminode trilobed with the mid-lobe smaller, 0.3–0.4 cm long, 0.4–0.5 cm wide, obtuse, mid-lobe mucronate.
Distribution. Western Mexico, state of Oaxaca.
Habitat. Grows lithophytically or terrestrially in xerophytic vegetation in seasonally dry conditions; up to 350 m.