Psidium oligospermum, the Galápagos guava or guayabillo,John C. Kricher (2006) Galápagos: A Natural History p. 34 is a small tree or shrub native to the tropical Americas, ranging from Mexico through the Revillagigedo Islands, Central America, Cuba, Puerto Rico, the Windward Islands, the Galápagos Islands, and South America to central Brazil and northwestern Argentina.
Description Psidium oligospermum is either a small tree or shrub that ranges up to in height and up to in diameter, with smooth, pinkish-grey bark. It has wide-spreading branches with dotted grey branchlets with reddish to white or yellowish "trichomes" or hairs. The branchlets tend to become more smooth at the edges and the bark more stringy, and the terminal branchlets and leaves are sometimes covered with a scurfy reddish bloom.Duncan M. Porter (1968) Annals of the Missouri Botanical Garden: Psidium (Myrtaceae) in the Galapagos Islands Vol. 55, No. 3, p. 368-371
Its leaves are opposite and elliptic to ovate, with the tips of the leaves being acute to acuminate. The base of the leaf is narrowly cuneate and is decurrent on the stalk of the leaf. The entire leaf is glabrous and is generally darker on the upper face and paler on the other side. The leaves are generally long and wide, and the petioles, or leaf stalks, are generally long.
The buds of Psidium oligospermum are pear-shaped or "pyriform" and connected to the base of the branchlet, extending about out. The bud is glabrous except for a minute hole at the apex with a few trichomes protruding outward.
Flowers are white, occur on branches of recent growth, and are relatively small, being in diameter. Its berries are spherical in shape and are glabrous except for ripples created from glands in the berries. The berries are yellow when mature and turn black or a reddish-brown when dried. They are in diameter and the "pericarp", or wall of the berry is about thick. The seeds are angular, dark, and long, and each locule contains several.
Habitat and ecology In the Galápagos Islands Psidium oligspermum is found on the islands Fernandina, Isabella, Pinta, Santa Cruz, and Santiago. It typically grows in arid lowlands and moist uplands.Conley K. McMullen (1991) Flowering Plants of the Galápagos p. 83
On Socorro Island in the Revillagigedo Islands, it is a canopy tree in upper-elevation Ilex socorroensis forest, with Ilex socorroensis, Guettarda insularis, and Sideroxylon socorrense.Ricardo Rodriguez-Estrella, Eustolia Mata, Laura Rivera, Ecological Notes on the Green Parakeet of Isla Socorro, Mexico, The Condor, Volume 94, Issue 2, 1 May 1992, Pages 523–525, https://doi.org/10.2307/1369224
Uses The berries of Psidium oligospermum are edible, reportedly with a slight taste of turpentine,Botanical label given by Ericsson (1947) and geese are reported to frequently consume the berries. The wood of the tree is used locally in the Galápagos Islands as fencing or a building material, but is not very hardy nor enduring.Julian Fitter, Daniel Fitter, David Hosking (2002) Wildlife of the Galápagos: Second Edition
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