The Poinsettia

The red cultivated poinsettia, which goes by Euphorbia pulcherrima, is a native of central Mexico and extends southward into South America. But Christmas poinsettias don't just come in red. Various cultivars have been bred from hybridizing the red poinsettia with a white species of poinsettia, known as Euphorbia cornastra, which is restricted to southwestern Mexico in the state of Guerrero. A full display of what cultivars have been bred, a good set of photos can be found here. But, aside from the bright red or white leaves, the wild plants don't look as much like the poinsettia in your home. In Mexico, they often become small trees! Here are a couple photos of E. pulcherrima.


Photo credit (left): victor_evg. Photo credit (right): Laura Uribe.

And you know those large red things on the plant, they're actually not petals. Those are actually modified leaves called bracts. And those small things in the center? You're getting closer, but those aren't technically flowers either. Those tiny things in the center actually each represent a cluster of flowers (inflorescences) surrounded by yet more modified leaves. Here's a closer look at a different plant:


Photo credit (left): Daniel Buenrostro.
Photo credit (right): Oliver Komar.

So, if those little nubs are clusters of flowers, what are the actual flowers? And what is this weird yellow round thing coming off of the flower cluster? What about the red bristly things that are yellow tipped? Before I answer those very good questions, I need to explain a little more about the overall morphology. Each cluster of flowers is enclosed by modified leaves (remember, these are called bracts) that are fused together to form a cup-shaped structure. This collection of bracts is collectively called the involucre. This surrounds the many flowers inside. And that yellow thing with liquid in it? That is a gland produced by the poinsettia. This produces nectar to attract insects. The entire thing is referred to as a cyathium (pl. cyathia). Here is a close-up of the cyathium of a related species (Euphorbia davidii):


Photo by Nathan Taylor.

The flowers may be male or female. Botanists refer to male flowers as staminate (because they produce stamens, which produce pollen) and female flowers pistillate (because they produce pistils which turn into fruits which inclose seeds). In poinsettia plants and Euphorbia in general, each cluster of flowers (remember, this is called a cyathium) produces one female (pistillate) flower in the center and many male (staminate) flowers surrounding the female flower. Both the female and male flowers are on there own stalks called peduncles or pedicels. I think it's time for a picture:


Photo by Nathan Taylor.

Well, now that you've been given this information, let's review. The red things are modified leaves called bracts, the nubs in the center are the flowers surrounded by modified leaves that are all fused together, and only within these modified leaves do you get the flowers. Complicated? Yep, Euphorbia is like that, but it makes things fun. If you want to learn the structures in even greater detail, I recommend checking out the information at Euphorbia PBI or this journal post discussing the internal morphology of the poinsettia cyathium specifically.

Now that you've hopefully learned something, it seems a bit unfortunate that you can't really see poinsettia in its full splendor in the US. However, it does have relatives here that are native. Perhaps the most similar to the Christmas poinsettia is a plant commonly known as wild poinsettia (Euphorbia cyathophora):


Photo credit (left): marytmn. Photo credit (right): Pam Kleinsasser.

Despite the similarity in name, the christmas poinsettia was not cultivated from wild poinsettia as some sources suggest. Another relative is sun spurge (Euphorbia radians), a less common, but no less interesting species:


Photo credit: nancygdi.

Most of the other related species are less showy. However, they are interesting in their own way. Take beetle spurge (Euphorbia eriantha) for instance. If you look at the photo below, you'll notice that the gland isn't cupped like the others. That's because it has narrow appendages that came out of the gland that covers it up. Why it does this is a bit of a mystery, but perhaps it helps to conserve water in the desert environments that it lives in.


Photo credit (left): ventura. Photo credit (right): Fred Melgert / Carla Hoegen.

Here's another. The scientific name is Euphorbia heterophylla, but the common names that it is given don't often make sense. For instance, Mexican fireplant is one and some others it shares with wild poinsettia (E. cyathophora). The reason is that E. heterophylla has often been confused with E. cyathophora (wild poinsettia). But I'll tell you the secret to distinguish the two. Euphorbia heterophylla will never have red on the leaves and may even have a bit of white. Even more importantly, do you remember the gland I talked about before? Well, in E. heterophylla, this is circular. In E. cyathophora, this gland is oval or oblong (at least, north of the southernmost part of Mexico). In E. heterophylla, they are circular. This is very important because E. cyathophora may lack the red coloration under certain conditions and some areas (particularly in central to north-central Texas). Here's a comparison of Euphorbia heterophylla and E. cyathophora glands so you won't forget:


Left: Euphorbia heterophylla, photo credit Nathan Taylor. Right: E. cyathophora, photo credit Jay Keller.

The last species I will discuss is actually somewhat weedy. It is called David's spurge (Euphorbia davidii) and depending on where you live, you might find it growing as a weed in your yard. It has two relatives that can be nearly indistinguishable from it, toothed spurge (Euphorbia dentata) and hairy-fruit spurge (Euphorbia cuphosperma). Here is a picture of David's spurge:


Photo credit (both): Nathan Taylor.

The differences between the three species are small and can be difficult to discuss without causing confusion, so I won't go into it here. However, what I want to point out is that you might be able to find one of these species growing wild nearby! I encourage you to go out and look for one of or all of these interesting species and compare it to the poinsettia in your own home. What are the similarities? What are the differences? When you're done, you might try to find another species of Euphorbia that I haven't mentioned and do it again. All of poinsettias close relatives can be found here if you want to learn more. Happy hunting!

Just for fun, here are a few more pictures of the native species in their late-season splendor.


Euphorbia davidii. Photo credit (both): Nathan Taylor.


Euphorbia eriantha. Photo credit (both): Nathan Taylor (second photo).

Other links:
Cyathium dissection and explanation
Euphorbia PBI cyathium explaination
Project recommended resource list

Posted by nathantaylor nathantaylor, December 22, 2018 00:04

Comments

Another helpful article. You amaze me Nathan. This is quite a Christmas gift to us iNat folks!

Posted by connlindajo almost 3 years ago (Flag)

Great post, Nathan!

Posted by janetwright almost 3 years ago (Flag)

Thanks for sharing.....

Posted by darrelbrown over 2 years ago (Flag)

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