What is a cyathium on a flowering plant? Does every plant have one? The answer is no, but some specialized plants do have them. Let’s find out more about these interesting plant parts.

What is a cyathium?

A cyathium is a whorl of bracts (modified leaves) fused together (involucre) with a flower in the middle.

Which plants have them?

You can see the pink bracts above on the Euphorbia milii or crown of thorns and below on the poinsettia. The small yellow colored flowers are in the middle of the bracts of the poinsettia Euphorbia pulcherrima and the entire thing is called a cyathium.

All euphorbias have cyathia and is the plural form of cyathium.

Euphorbia pulcherrima or poinsettia and Euphorbia milii or crown of thorns, both shown here, are two houseplants most are familiar with. Did your mom or grandmother have a crown of thorns? They are plants that have been around for a long time.

Parts of a cyathium

A cyathium has an involucre (in·​vo·​lu·​cre), which are the bracts fused together surrounding the true flower. The colored bracts attract pollinators in their natural habitat.

Many assume those bracts are the flower petals. We pick our poinsettias according to the color of the “flowers”, but those are the bracts or modified leaves of the plant and the true flowers are the yellow parts in the middle of the involucre.

Cyathium flowers

The flowers can be male, female, or both and according to Daryl Koutnik, Ph.D. his Euphorbia bupleurifolia had female flowers one year and male the next. One year it had both and he collected seed. (This information is from the book Euphorbia, volume 2, 1984.)

Below are pictures of the crown of thorns cyathia with pollen showing on the male flowers.

Below is a crown of thorns seen at the Key West Butterfly and Nature Conservatory and you can see a butterfly is nectaring on its flower.

Female Flower

The poinsettia below has many male flowers and one female flower, which you can see on the left. It is protruding above the flower below so this would not be the poinsettia to pick because it won’t last as long.

When the flowers die, the plant assumes it has done its job, which was to make flowers, get pollinated, and set seeds. If the flowers have done that, the plant begins to decline.

Euphorbia francoisii

I have a few euphorbias other than the crown of thorns and the poinsettia because I collect them.

I have Euphorbia francoisii, which is a small plant with a caudiciform or swollen stem. The flowers are tiny and only approximately 1/4″ across.

Euphorbia millotii

Another interesting euphorbia I have is Euphorbia millotii and its involucre is a bit bigger than the one above.

I love the green color of the bracts, but as the flowers finish, they turn a pinkish color and then dry up and fall off.

I love learning new things about plants I have in my home so I hope you found this interesing. Did you know the colored parts of your poinsettia are bracts, not flower petals?

Did you know there were so many euphorbias? The family has many interesting plants. Do you have any other than the poinsettia? Tell me in the comments.

Have a great week, plant friends!

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