From Mori et al. (in review)

Gustavia clade (Mori, 1979b)—the growth form of species of Gustavia is variable; some species are pachycauls with very robust stems (greater than 10 mm diam.) that can be unbranched (e.g., G. monocaulis) or with several orders of branching (e.g., G. superba); some species are leptocauls with medium-sized leaves, robust stems (from 5 to 10 mm diam.), and densely ovate crowns; and others are leptocauls with more slender stems (usually 5 or less mm diam.), relatively small leaves and dense but not necessarily ovate crowns. The pachycaul species usually have very large leaves that approximate the lengths, but do not surpass those of species of Grias. The trunk is cylindrical to the ground in all species. Although eight petals (Fig. 3A) are most common in this genus, there can be as few as six (e.g., all species of the G. hexapetala group) or, in one case, as many as 12(18) (G. romeroi). The androecia of all species of Gustavia are actinomorphic (Fig. 3A) and possess a thin, broad staminal tube (ca. 2 mm thick and 10 mm or more in diameter). The filaments arise from the rim of the staminal tube and are much longer than those of any other neotropical Lecythidaceae; however, the paleotropical Barringtonioideae have similar androecia. Gustavia is the only genus of Lecythidaceae that has poricidal anthers (Fig. 3F), which suggest buzz pollination (Boeke & Mori, 1987), pubescent ovary summits (the simple trichomes are septate), anthers greater than 1mm long (Fig. 3F), apically expanded placentae (Fig. 3G), and embryos with fleshy, irregular-shaped cotyledons. In addition, the fruits of Gustavia are indehiscent and globose in shape (Figs. 3C, 3E) and the seeds are of two major types: those with relatively thick seed coats with funicles surrounded by a thick, yellow, contorted aril (e.g., G. augusta, Fig. 3B) and those with membranous seed coats with inconspicuous funicles surrounded by a tubular aril (e.g., G. nana subsp. rhodantha and G. superba).