Porteres in Bouriquet (1954) describes 110 species of vanilla, distributed in the tropics of both the world and the New World. They belong to the orchid family, Orchidaceae, which is the largest family of flowering plants, with about 700 genera and 20,000 species. The Orchidaceae comprise a very natural, distinctive and highly advanced group of monocotyledons. They are perennial herbs which are widely distributed throughout the world with the greatest number in the tropics.
They exhibit a wide range of life form and have terrestrial, climbing, epiphytic and saprophytic species. Apart from the large number of ornamental species which are grown for the flowers, vanilla is the only genus which has species of economic importance. The correct name of the commonly cultivated vanilla of commerce is Vanilla fragrans (Salisb) Ames, Syn V. planifolia Andrews, Epidendrum vanilla L. Myrobroma fragrans Salisb.
Two other species are occasionally cultivated, but yield an inferior product, they are:
1) V. pompana Schiede, West Indian Vanilla, which occurs wild in southeastern Mexico, Central America, Trinidad and northern South America. It is cultivated to a small extent in Guadeloupe, Martinique and Dominica. It resembles V. fragrans, but the leaves are larger, being 15 - 30 cm long and 5 - 12 cm wide. The greenish yellow flowers are larger and more fleshy, with perianth lobes up to 8.5 cm long. The lip has a tuft of intricate scales, instead of hairs in the center of the disc. The cylindrical pods are shorter and thicker, being 10 - 17.5 cm long and 2.4 - 3.3 cm in diameter.
2) V. tahitensis J.W. Moore, Tahitian vanilla, indigenous to Tahiti and cultivated there and in Hawaii. This species is less robust than V. Fragrans with more slender stems and narrower leaves which are 12 - 14 cm long and about 9 - 10 mm wide, tapering towards each end.
V. fragrans is a fleshy, herbaceous perennial vine, climbing by means of adventitious roots up trees or other supports to a height of 10 - 15 meters. In cultivation it is trained to a height which will facilitate hand pollination and harvesting.
Long, whitish, aerial, adventitious roots, about 2 mm in diameter, are produced singly opposite the leaves and adhere firmly appressed to the support up which the plant climbs. The roots at the base ramify in the humus or mulch layer. An endotrophic mycorrhiza is present. The outer parchment-like sheath or velamen is rather poorly developed.
The long, cylindrical, monopodial stems are simple or branched, and are succulent, flexuouse and brittle. They are 1 - 2 cm in diameter and are dark green and photosynthetic with stomata. The internodes are 5 - 15 cm in length.
The large, flat, fleshy, subsessile leaves are alternate, oblong-elliptic to lanceolate, and are 8 - 25 cm long and 2 - 8 cm broad. The tip is acute to acuminate and the base somewhat rounded. The veins are numerous, parallel and indistinct. The petiole is short, thick, and canalized above.
The stout inflorescences are axillary, usually simple, and only rarely branched. They are usually borne towards the top of the vine and are 5 - 8 cm long, with up to 20 - 30 flowers, but more usually 6 - 15 opening from the base upwards, generally with only 1 - 3 flowers open at one time and each lasting one day. The rachis is stout, often curved, and 4 - 10 mm in diameter. The bracts are rigid, concave, persistent, 5 - 15 mm long and about 7 mm wide at the base.
The large, waxy, fragrance, pale greenish yellow flowers are about 10 cm in diameter and are fugacious. The pedicel is very short. The inferior, cylindrical, tricarpillary ovary is often curved, 4 - 7 cm long and 1 - 1.5 cm wide. The two upper petals resemble the sepals in shape, but are slightly smaller. The lower petal is modified as a trumpet-shaped labellum or lip, which is shorter than the other perianth lobes and is 4 - 5 cm long and 1.5 - 3 cm broad at its widest point. It is attached to the column which it envelops. The tip of the lip is obscurely three-lobed and is irregularly toothed on its revolute margin. There are longitudinal verrucose darker colored papillae forming a crest in the median line and with a tuft of hairs in the middle of the disc. The column or gynostemium is 3 - 5 cm long and is attached to the labellum for most of its length. It is hairy on the inner surface, bearing at its tip the single stamen containing the two pollen masses or pollinia covered by a cap, and below is the concave sticky stigma, which is separated from the stamen by the thin, flap-like rostellum.
FRUIT The capsule, known in the trade as a bean, is pendulous, narrowly cylindrical, obscurely three-angled, 10 - 25 cm long and 5 - 15 mm in diameter. It is aromatic on drying, containing, when ripe, myriads of very minute globose seeds, about 0.3 mm in diameter, which liberated by the capsule split longitudinally. In commercial production the capsules are harvested before they are quite ripe.
POLLINATION The flower is so constructed that self-pollination of the individual flower is impossible, unless hand-pollinated, due to the separation of the stamen from the stigma by the rostellum.
In Mexico and Central America, where vanilla is indigenous, some of the flowers are pollinated by bees of the genus Melapona. Nectaris secreted by the base of the lip and the flowers are sweet-scented. Hummingbirds have been observed visiting the flowers and it has been suggested that they may also be pollinating agents. Elsewhere, hand-pollination is carried out. In Puerto Rico, natural pollination was about 1 percent (Childers and Cibes, 1948). The method of hand-pollination was discovered by Morren in Liege in 1836, and Edmond Albius, a former slave in Reunion, discovered a practical method of artificial pollination in 1841, which is still used.
V. fragrans usually flowers only once a year over a period of about two months. In Mexico this is usually in April and May and in the Malagasy Republic, Reunion and the Comoros Islands between November and January. As already stated, the flowers open from the base of the raceme upwards and usually one, or more rarely two or three flowers are open on the inflorescence at one time.
The flowers open early in the morning; they are receptive for eight hours and wither the following day. Fruit-set is highest when pollination is done early on bright mornings following rain. If fertilization has been successfully achieved, the flowers remain on the rachis; if unsuccessful, the flowers drop off in two or three days. Thus it is possible to judge the number of fruits which have set and to discontinue pollination when the desired number have been obtained.
Hand-pollination is done with a splinter of bamboo or other material about the size of a toothpick. The flower is held in one hand and the labellum is pushed down with the thumb releasing the column.
The stamen cap is removed by the stick which is held in the other hand which exposes the pollinia. Then the flap-like rostellum is pushed up under the stamens with the stick and, by pressing with the thumb and finger, the pollinia are brought into contact with the sticky stigma to which the pollen mass adheres.
When fertile cross-pollinated seed is required, it is, of course necessary to obtain the pollinia from another plant.