Poinsettia flower hide-and-seek

December 4, 2009

Poinsettia plants (Euphorbia pulcherrima) have become a traditional winter holiday decoration. If you have one available, try to find its flowers. It isn’t an easy task. While every species of flower is a variation on the basic theme of flower parts, the poinsettia and other members of the euphorbia family are particularly unusual.

First, I should define what I mean by a flower. Botanically it is the reproductive structure of a flowering plant. A “complete” flower – one that has all the usual parts – has a calyx made of sepals, a corolla made of petals, stamens, and a pistil. Not all these parts are necessary for the flower to function – that is, to make a fruit and seeds. To be a flower, the minimum requirement is at least one stamen or one simple pistil (carpel). All those other structures just enhance the function of the stamen or the pistil. The stamen produces pollen, which holds the sperm cells that will fertilize the egg cell and endosperm in the ovule. If the ovules are fertilized, the ovary of the pistil matures into a fruit with seeds inside.

If you want to figure out the poinsettia’s flowers, it really helps to see the developmental sequence. Poinsettias are usually sold when the flowers are fully formed, so I’ve provided some photos of earlier stages to help you see what’s what.

In mid-October, these poinsettia plants are all green. They haven't started forming bracts.

First, let’s take a look at the parts are NOT flowers. Hint: look for the most conspicuous and showy parts. The colorful petal-like parts are simply modified leaves that help attract pollinators. They are called bracts, and they develop when the days are short in the fall. The photo

Only ten days later, red bracts are starting to form.

sequence of poinsettias in a greenhouse shows that the bracts are leaves that are colored. Bracts have the same vein pattern as leaves and if you look at the stem, you will see that they have an alternate arrangement – one bract at each node – instead of the whorled arrangement that petals usually have. 

The bracts have an alternate arrangement on the stem - one per node.

 

Look at the photo below that shows a cluster of small green rounded structures in the center of the bracts. Are they the flower buds? No, they are developing inflorescences. An inflorescence is a group of flowers on a single stem. The botanical term for this inflorescence is a cyathium, which comes from the Greek term for a cup. As these structures mature, they become cup-shaped. The rim of the cup takes on a color, yellow to red, depending on the variety. One or two structures that look like yellow or orange lips grow on the top of the cup. While most nectar glands are in a single flower, these reward pollinators for visiting the whole inflorescence.

The little green structures in the center are the early stages of the inflorescences, the cyathia.

Each cyathium holds either stamens, a pistil, or a mixture of them both. Normally when you see stamens and a pistil, you are looking at parts of one flower. Here each individual stamen or pistil is a separate flower with its own separate stem. You can see this most clearly on the pistils. Their stems elongate and push them out of the cyathium as they mature. Many of the poinsettias sold commercially seem to have only staminate flowers. You can see the dusty yellow pollen on the anthers at the ends of the filaments. The pistils are three-carpellate – composed of three fused carpels (aka simple pistils). The style of each carpel curls open and splits in two as it matures. When the pistil first forms, you may see only the styles and stigmas. As it grows, you can see the whole ovary and its stem hanging out of the cyathium.

These cyathia have only stamens. Note the yellow anthers and red filaments. The yellow nectar glands are near the top of the cups.

In this variety, the styles and stigmas are red. The stamens are yellow. The pistils have three styles that each split in two.

Even if your poinsettia has pistillate flowers and its ovary grows, don’t expect seeds. Most hybrids with those large colorful bracts are no longer fertile. They are propagated from cuttings. Poinsettia growers seldom grow plants from seeds. If they did, they would get a range of bract colors and maturity times, rather than all the plants looking the same. In fact, poinsettia seeds are hard to find. I found only the wild type in a recent search.

The ovaries of the pistillate flowers have enlarged. Their stems have pushed them out of the cyathia.

After your poinsettia has bloomed, the bracts remain even as the cyathia drop off. You can keep it and allow it to grow. In the spring you should cut back the bracts to keep the stems from becoming long and scraggly. This allows new leaves to form. If you want the plant to form bracts and flowers again, it will have to be kept in the dark for about 15 hours a day until the bracts are colored. For more information, see http://aggie-horticulture.tamu.edu/extension/newsletters/hortupdate/novdec00/art1nov.html  

Poinsettias are rumored to be poisonous. They certainly should NOT be considered edible, but their milky sap is only mildly irritating. Finally, my thanks go to TaTonka Farms of Conifer, Colorado for allowing me to photograph their beautiful poinsettias throughout the growing season.  Happy holidays!

This variety has variegated bracts.

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