TYPE: COLOMBIA: Rib Atabapo, occasional along drowned river margin between San Fernando de Atabapo and Cacagual, 150 m elevation, 18 Nov., 1953, Bassett Maguire, John J. Wurdack & George S. Bunting 36261 (NY).
Vernacular name: "Majagua."
The genus Heteropetalum belongs to the North Brazilian flora; it is indigenous in the Rio Negro and Rio Casiquiare districts, but also occurs in the adjoining parts of Venezuela and Colombia. The two species so far known—H. brasiliense and H. spruceanum—are very closely related, and it might possibly be more correct to unite them into one species. The genus is characterized by the greatly reduced size of its outer petals; these are strikingly smaller than those of the inner whorl, and in the species so far known very like the sepals. In H. brasiliense the sepals and outer petals are of equal size and short-pointed, but in H. spruceanum they are protracted into narrow tips. In our variety longipetala, which in respect of its vegetative parts is closely related to H. spruceanum, the outer petals—in the rest of the genus short and sepal-like—are nearly 2 cm long; the fact that they are directed outwards and arched backwards in a peculiar way is very striking.
The great reduction of the outer petals into sepal-like shapes which characterizes the genus Heteropetalum, has previously led some authors to attach the genus to the Miliusa group of the Annonaceae. In a paper published in 1942 (in Arkiv for Botanik) the present writer has, however, tried to show that this character is not particularly important from a systematic point of view, and that in other respects this genus fits well into the Guatteria section of the Annonaceae system. The fact that a type like the variety described here has been found to have long outer petals quite different from the sepals provides additional support for the opinion that the relationship of Heteropetalum to the Miliusa group is not very close.