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An Excerpt from Jim Conrad's
Naturalist Newsletter

Wild Poinsettia, Mexican Fireplant, Painted Euphorbia, Fire on the Mountain --  EUPHORBIA HETEROPHYLLA

from the June 27, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
WILD POINSETTIA

Christmas Poinsettia's technical name is Euphorbia pulcherrima. A very common, knee-high herb presently flowering here now, often as a weed along roadsides, is the closely related EUPHORBIA HETEROPHYLLA, which goes by several English names, especially Wild Poinsettia, Mexican Fireplant, Painted Euphorbia, Fire on the Mountain and Paint Leaf. You can see it above.

Maybe you notice similarities between this plant and the Christmas Poinsettia, especially in the similar leaf shape, how the leaves cluster at the stem's top, and how the cluster's middle leaves are colored toward their bases. Of course Christmas Poinsettias are red where the plant in the picture is white. However, this is a very variable species and sometimes the leaf bases are red.

A close-up of the flowers at the cluster's center is shown below:

Wild Poinsettia, Mexican Fireplant, Painted Euphorbia, Fire on the Mountain --  EUPHORBIA HETEROPHYLLA --  cyathia

Using the black ants for scale (and they're small ants) you can see that we're seeing things at very close range. And the "flowers" are classic Euphorbia "flowers," which means that their structure is surreal.

First, the green doughnut thingies. Notice that some of them are glistening, and that an ant appears to be feeding at one. The glistening substance is nectar, so the green doughnuts are nectar glands exuding nectar the ants like. One guesses that the ants, in return for the nectar, protect the plant from herbivores who may come nibbling. And that white and green nymph at the lower right -- who knows how it fits in?

Each green nectar gland is associated with one urn- shaped structure peculiar to Euphorbia and closely related genera. The urn-shaped thing, which is about 1/8th inch across (2.5 mm) is called a cyathium. Cyathia are not flowers, but rather clusters of flowers, almost as seen among composite flowers in the Composite Family. A single cyathium is shown below:

EUPHORBIA HETEROPHYLLA -- cyathium

Three main items are visible in that picture. The whitish, flowerlike, urn-shaped cyathium lies a little to the picture's left. A yellow-green nectar gland arises on the cyathium's left. Unseen inside the cyathium reside a few -- five or so -- unisexual male flowers that are so streamlined or reduced that they consist of no more than one stamen arising from its pedicel, the pedicel being a flower's stem.

The green, spherical item to the cyathium's right is the female flower, which consists of the green ovary atop a slender pedicel anchored at the cyathium's bottom inside. In other words, the cyathium's single female flower hangs outside the cyathium. There's not enough room for it inside. The white, elongate things at the green ovary's right are style segments with stigmatic areas on which pollen grains germinate.

All Euphorbia flowers are structured more or less in the same way, in cyathia, though the cyathium of each species is a little different from the cyathium of every other species -- and worldwide over 1000 Euphorbia species are recognized, so just think about it. In 2006 we met another flowering "Wild Poinsettia," a Euphorbia, Euphorbia cyathophora, so you can see how that differs from this species at http://www.backyardnature.net/yucatan/poinsett.htm.

Our current Wild Poinsettia is native from California to eastern Texas through Mexico and into much of Central America. As an invasive weed it's spread to South and Southeast Asia, where in India and Thailand it invades cotton fields and other disturbed environments.

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