from the September 19, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO; limestone bedrock, elevation ~39m (~128ft), ~N20.676°, ~W88.569°
TROPICAL GRAPES FLOWERING
A Cissus species with trifoliate leaves and yellowish inflorescences, is shown below:

CISSUS TRIFOLIATA

A close-up of this species' flowers, this time with a blossom bearing four turned-back, yellow petals and four anther-bearing stamens, with no nectar globule, is shown below:

CISSUS TRIFOLIATA, flowers

NOTE: In 2016 more information is available on the Internet, and this seems to be CISSUS TRIFOLIATA, a common and variable species widely distributed from the southern US throughout Mexico and the Caribbean south through Central America and most of tropical South America. See Jerzy Rzedowsky's excellent Spanish-language treatment of the Grapevine Family, the Vitaceae, in the "Flora del Bajío y de Regiones Adyacentes," freely downloadable in PDF format here.
I'm guessing that this is ... {see note at right}.

Really I hadn't realized that Cissus blossoms are so pretty and interesting. To discover this, first I had to get so blind that I could no longer focus up close or use my hand lens well, and thus had to photograph small things and see them on my laptop screen!