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Orchidaceae family

Vanilla planifolia
, Bot. Repos. 8: t. 538.  1808.

Vanni. Vanilla.
Indigenous, escaped (?) quite rare vine around forest edge. 
Antigua, Guadeloupe, Dominica, Martinique, St. Lucia, Grenada, rest of West Indies, Mesoamerica, South America.   

As this species is frequently spread on purpose it is difficult to know whether this species is native to St. Lucia. I have never observed this species in really isolated areas.