An Excerpt from Jim Conrad's
Naturalist Newsletter

ONCIDIUM CARTHAGENENSE

from the July 18,, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
MULE EAR ONCIDIUM

Deep in the forest atop a low mound you might not guess once was a Maya structure, one never excavated so maybe still holding secrets, a little orchid in full bloom grew in dim light on the side of a tree. That's it above.

It's ONCIDIUM CARTHAGENENSE, sometimes placed in the genus Tricocentrum, widely distributed from Mexico and the Caribbean to northern Brazil. Because of the leaf's shape and because often two leaves arise together, some of its names translate to "Mule Ear." Oncidium is a big, well-known genus including many species fancied by orchid growers, so a good English name for the species is Mule Ear Oncidium.

Mule Ear Oncidiums are typical of the genus. Back in Chiapas we met another species, Oncidium phymatochilum, and you might enjoy comparing it, which also bears many small, pinkish flowers in a diffuse inflorescence larger than the leaves, with ours. See http://www.backyardnature.net/chiapas/o-o-phym.htm.

We also met Oncidium cebolleta in Querétaro. It's seen at http://www.backyardnature.net/q/oncidium.htm.

Many orchid genera produce flowers at the end of a stem arising from the plant's center. Oncidiums are different from them in that they produce special inflorescences, or flowering heads, on stems arising apart from the tufted leaves. Also, the stems of inflorescences of many genera keep growing year after year, but Oncidium inflorescences do all their growing in a single year. Many orchid species sprout their inflorescences and leaves from conspicuous, more or less egg-shaped, water-storing, photosynthesizing parts of the stem known as pseudobulbs. Oncidiums don't have pseudobulbs.

You can see front and side views of a flower below:

ONCIDIUM CARTHAGENENSE, flower

Among the flower's most easily observable features distinguishing it as an Oncidium blossom is that its lower lip is well attached to the rest of the flower -- not movably jointed. Also, the lip bottom flairs making it somewhat fan-shaped. The lips of many Oncidiums are three-lobed, but this species is only two-lobed. Finally,where Oncidium blossom lips attach to the rest of the flower, there are wart-like "cushions" or "callosities," showing up as a darker purple in the frontal picture at the left, and as a forward-projecting bump on the side picture at the right.

So, if you're halfway interested in neotropical orchids, the genus Oncidium is one of a handful you really ought to know, and the above distinguishing features for the genus are worth keeping in mind.