Handbook of Vanilla Science and Technology

The most recent systems of classification for subfamily Vanilloideae (Chase et al. 2003; Cameron 2003) considers 15 genera divided into 2 tribes: Vanilleae and Pogonieae. A phylogenetic reconstruction of the subfamily is shown in Figure 14.1. Within tribe Pogonieae are both tropical and temperate species. Duckeella contains one or possibly two species from Venezuela and northern Brazil. The genus produces long linear leaves and bright yellow flowers that rise above wet grassland and savanna habitats. It may occasionally be found rooted in mats of floating vegetation. Closely related is the genus Cleistes (> 30 species), which contains species mostly from tropical South America. These plants are also found in open savannas, but may survive predictable periods of drought by entering an annual state of dormancy and storing food within underground tubers. One species of this genus, Cleistes divaricata, is native to the southeastern United States. Detailed systematic studies of the tribe indicate that this species might be better treated as a separate genus (Cameron and Chase 1999). Also native to the United States is the genus Isotria, which contains two species. The flowers of Isotria are typical of other vanilloid orchids, but the arrangement of leaves into a single whorl is distinctive. These plants are spring ephemerals that emerge and reproduce quickly within their deciduous forest habitat before the tree canopy closes fully during the summer months. They also reproduce asexually via long underground roots, and are strongly mycorrhizal. The fourth genus within Pogonieae is Pogonia, which exhibits an interesting geographic disjunction between eastern North America (with one species, P. ophioglossoides) and eastern Asia (with 3-5 species). These plants are found most commonly in acidic bogs, around the edges of lakes, and within wet savannas. The fruits of all Pogonieae are true capsules and there is no evidence that they produce vanillin. Because of their dormancy requirements and strong relationships with mycorrhizal fungi in the soil, they are rarely cultivated and should be left undisturbed in their natural habitats.

Fig. 14.1 Cladog ram depicting the phylogenetic relationshipsamong genera ofVanilloideae based on a combination of nuclear, mitochondrial, and plastid DNA sequence data. The subfamily is divided into two tribes: Pogonieae and Vanilleae. Note that Vanilla shares a common ancestor with a clade of four genera including Galeola and Pseudovanilla.

14.2.2 Tribe vanilleae

In addition to Vanilla itself, the tribe Vanilleae contains eight other genera that are exclusively tropical or subtropical, but wide in distribution, and diverse in morphology. Two of these, Eriaxis and Clematepistephium, are endemic to the isolated Pacific island of New Caledonia, 1,400 km northeast of Australia. Both genera are monotypic, containing only a single species each. Eriaxis rigida is an upright herb with stiff leathery leaves that grows within the full sun, metal-rich, nutrient poor soils of New Caledonia’s maquis. The pink flowers of Eriaxis are highly attractive, but the genus is nearly impossible to cultivate outside of its native habitat. Its close relative, Clematepistphium smilacifolium, could not be more different. This species is restricted to the dense shade habitats of the New Caledonian rainforests, and grows as a climbing vine. In contrast to Vanilla, however, the species produces no aerial roots, but instead climbs by means of twining around the trunks of small trees. Its large, leathery leaves exhibit prominent venation patterns that are reticulate (net-like) rather than exclusively parallel as is the norm for monocotyledons (Cameron and Dickinson 1998). Fresh flowers of Clematepistephium are primarily green in color, but have been seen by very few people.