Orchidaceae

Habit: Plant a robust, succulent vine climbing up to 30 meters, one to many-branched, adhering by aerial roots. Stems green, thick and succulent with irritating sap, 0.9 – 2.3 cm in diameter; internodes 9 – 13 cm for climbing individuals attached to host. Aerial roots produced from each node; aerial roots short, stout, white, unbranched; ca. 0.5 cm in cross-section, ca. 3 – 5 cm long. Terrestrial roots emerging from lower nodes and entering superficially into substrate; branched and extensive.

Leaves: Leaves alternate, sessile, succulent, produced from each node opposite to roots, held at 30 to 60 degrees below horizontal, highly variable in size and shape, ca. 3 x 6 - 13 x 30 cm; blade elliptic to orbicular, apex acute to rounded, margin entire, texture smooth.

Inflorescence: Inflorescences racemose, axillary, fleshy, green, sessile, ca. 1 to 20-flowered; bracts ca 1 x 1.5 cm.

Flowers: Flowers resupinate, tender, fleshy, ca. 15 cm wide, arranged spirally, variably fragrant, remaining attached to ovary if successfully pollinated; pedicel green, persistent on maturing fruit; buds green; dorsal sepal yellow, elliptic-oblanceolate, reflexed backwards, ca. 1.5 x 9.4cm; lateral sepals yellow, reflexed, ca. 1.5 x 8.5cm; petals yellow, with thickened midvein on dorsal surface, reflexed, ca. 1.1 x 9cm; lip uniformly yellow except for a characteristic white triangle on ventral surface, opening reflexed and caniculate to callus, somewhat saccate when fresh, flabellate when flattened, margins crisped, fused with column for ca. 5 cm, darker yellow lines connecting lip edge and callus ca. 2.2 cm long and 0.3 cm wide; callus well-developed and robust, with slight sinusoidal curve, ca. 0.8 x 0.6 cm; column hirsute on ventral surface near rostrellum gradually becoming glabrous, ca. 6.3 cm long, 0.4 cm wide; rostrellum broad, thick, yellow.

Fruit: Fruit fleshy, triangular in cross-section, green when immature, ca. 2.6 x 15 cm, extremely oily and slow drying, brown and fragrant when dehiscent.

Distribution: The true pompona, V. pompona Schiede, is a narrow endemic on the Mexico/Guatemala border and all records of its existence in South America are wrong. Vanilla pompona subsp. grandiflora is widely distributed from Honduras to Bolivia (Soto Arenas pers com). In Madre de Dios it is mostly restricted to open, flooded savannah habitat within M. flexuosa wetlands, characterized by deep, permanently saturated histosols and minimal water level fluctuations. While the vine is robust and resilient to frequent disturbances caused by falling palm fronds, its roots are vulnerable to even short periods of inundation. Within its habitat it is generally restricted to small, elevated areas formed by the accumulation of organic matter. It often forms dense populations over extensive areas where conditions are favorable.

Ecology: The flowers of this species open singly, occasionally doubly during cool weather spells and generally last approximately eight hours, opening as early as 3:00 AM. The maximum number of flowers observed on a single individual was 200. Flowering phenology is bimodal, with two rather short flowering episodes of approximately 30 days in April and September/October. Significantly more individuals flower during the September/October season, probably in response to the long dry season beginning in June/July. During the flowering season, individuals tend to flower more profusely immediately following small-scale (5 C° or less are sufficient) temperature reductions (i.e. friajes). Flowering responses to temperature are facilitated by determinant racemes with buds almost fully developed prior to the initiation of the flowering season. Several of the apical buds may be poised to open during unpredictable cool spells. The response is general, and immediately following friajes the entire population may not flower for one to several days.
Flowers produce a “sweet” fragrance, but this is sometimes barely perceptible and the potency varies between individuals. Particularly fragrant flowers attracted two species of Euglossine bees, Euleama meriana and Euglossa imperialis. Several individuals of both species may repeatedly circle a single fragrant flower for several minutes, often demonstrating antagonistic behavior. The smaller, and generally more numerous Eug. imperialis is the more aggressive species, however its small size probably precludes it as a potential pollinator. We have observed only Eul. meriana to pollinate flowers on two occasions. In both cases a solitary bee entered the floral tube for two to four seconds and with no delay departed along a straight line track. Collection of fragrance of floral origins has never been observed, suggesting that flowers are deceptively pollinated (see [(Lubinsky, Van Dam et al. 2006)] for a more detailed description).

Pollination rates are extremely low, measured at less than one percent in September 2005 (Table 1). The number of fruits per individual ranged from one to four, the great majority of fruiting individuals producing only a single fruit during any given season. Fruits mature and begin to dehisce approximately five to nine months after pollination and open slowly over a period of approximately two weeks, revealing hundreds of thousands of tiny seeds. Mature (dehiscing) fruits have a sweet vanilla aroma. Fruit fragrance is attractive to a limited number of bee species, interestingly including the two Euglossine species attracted to floral fragrance, Eul. meriana and Eug. imperialis. Both species demonstrate typical fragrance collection behavior (see [(Dressler 1968)] for a detailed description of fragrance collection behavior) and may remain at a single fruit for over a 15 minutes. Other common bee visitors include a single species of stingless bee (Trigona sp.), and one species of Carpenter bee (Xylocopa sp.). The Trigona sp. has been observed to collect and fly away with small packets of twenty seeds on their hind tibia. Although not confirmed, the tiny seeds embedded in the fragrant, oily mesocarp may also be intentionally collected by the Euglossine bees during fragrance collection. Nothing is known about the fate of these seeds once collected, however we have often observed Trigona bees dropping their seed packets. Occasional herbivory of dehiscent fruits suggests that larger vertebrates, such as bats or locally abundant marsupials, may also be important dispersers, but we have never made direct observations.

Seedling emergence is distinctly seasonal. Most seedlings surface in May, coinciding with the initiation of the dry season. However, seedlings are extremely rare and most reproduction is through vegetative propagation. The vine is extremely clonal and vigorously growing; monthly growth rates often exceed one meter. Frequent disturbances due to falling palm fronds often topple vines fragmenting them in the process. Fragmented sections readily sprout vegetative buds and in this manner form locally dense, clonal populations.

This Vanilla species demonstrates a degree of myrmecology, especially during bud maturation. Small Azteca ant colonies anticipate the production of sugars exuded at the abscission layer of immature buds by building colonies in the small, circular depressions housing the axillary meristems.

Notes: Vanilla pompona subsp. grandiflora is a member of the American clade of fragrant Vanilla. New molecular data suggest that the “true” V. pompona of Mexico is nested within the variable V. grandiflora, but the older name, V. pompona, is retained.