The Poinsettia Mystery

I was a biology major in college. During that time I was a student employee and I got stuck with the mind-numbingly boring and frustrating task had the opportunity and challenge of gluing hundreds of friable, sometimes paper-thin plant specimens into specimen books. I also took a botany course, but it was only one semester long and I believe that the most I’ve retained from that class is an understanding of the differences between deciduous and coniferous trees. Actually, that was one of my big SCI-ENCE! moments. I remember being hugely impressed with needle-bearing evergreens’ adaptive strategies for surviving in nutrient-poor soil, and arid and low-sunlight conditions. Rah! Rah! Evergreens!

Anyway, that’s just a side story. The point is, I’m not a botanist, but I do imagine myself to be a bit of a naturalist and appreciator of ecological science. And what I’m…. Well… Hmmm… What I’m trying to say is that I learned something new, something that’s probably common knowledge and I’m a little embarrassed to have not known this, especially having studied a bit of botany. But on the other hand we all make assumptions about little things that don’t have much to do with out day-to-day needs or experiences, and it’s neat to have those misconceptions or ignorances cleared up. Here’s the story:

We, like many work offices, acquired a poinsettia over the holiday season. You know, one of these:

Image Source

And as is the fate of so many under-appreciated and ignored office plants this one didn’t get nearly as much water as it needed. Eventually all of the green leaves blackened and curled and fell off to their final resting place on the dry soil. For some reason, after the last of the green leaves dropped, the officemates and I were spurred into action. We became poinsettia paramedics, determined to bring this abused plant back from the brink of destruction to which we had driven it.

As we were considering the poinsettia several days later I had this conversation with my office mate, R.

Me: Isn’t it weird that the leaves fell off  before the petals did?

R: I think the red ones are leaves too, not petals. 

Me: No way! Why have two different types of leaves on a plant? And leaves have chloroplasts, and don’t those cause the leaves to be green?

R: (A PhD in virology and rather far removed from his own botany course) I don’t know. Maybe. But I don’t think it’s necessary to be green if you  have chloroplasts.

Me: I don’t know either. I bet the concentration of chloroplasts is less in the red parts than in the green leaves. I wonder how long the plant is going to live without the green leaves. Will it be able to produce enough energy to survive?

R: Hmm. [goes back to his computer]

Me: You know, if only there was someplace where we could easily look all of this stuff up.

R: [Playing with a ROC curve and only half listening to me ramble anymore] Have at it.

To the interwebs! As I knew it would be, the answer was immediately presented:

From the website

At first glance one might come to the erroneous conclusions that all leaves are green, and that which is green in nature is a leaf. While often this is the case, there are numerous exceptions. Plant organs are green because of the presence of chloroplasts in the cells near the surface, which reflect green light and absorb other wavelengths as a source of energy for photosynthesis. Certain cells in many leaves contain these organelles , but chloroplasts are also found elsewhere in other organs, such as the stems of cacti of the desert and twigs of sassafras trees in the deciduous forest. In addition, flowers such as the head of broccoli, and fruits such as watermelons also contain chloroplasts.Conversely, many leaves are not green. The winter holiday season brings potted poinsettia plants into many homes. The bright red or pink organs on these plants are not the flowers; they are specialized leaves called bracts, with cells that contain so much pigment that the limited amount of chlorophyll in the chloroplasts is obscured from view. Some poinsettias are white; usually a close look reveals that they have a green tinge due to the presence of a few chloroplasts. Poinsettias do, however, produce flowers. They are less conspicuous small, round yellow and green organs nestled at the apex of the stem, surrounded by the colorful modified leaves.

So – one mystery solved! But why bracts? Wikipedia says that the colorful red leaves serve the function of attracting pollinators. And an article on e-how provided some fascinating insight on why the poinsettia in our office is the way it is:

Why Colored Bracts Form: In nature, the poinsettia plant is deciduous and drops its leaves in the winter. Some of the leaves change color before they fall. Growers carefully control the light exposure of poinsettias cultivated for the home market to mimic this natural cycle and the topmost leaves, or bracts, turn color in response.

Special Breeding and Cultivating: Although all poinsettias have descended from the tall perennial Mexican plant, plant cultivators have bred and selected plants over decades to produce poinsettias with a height and appearance appropriate for inside decoration. In addition to shading leaves to induce color change, growers pinch the plants to produce side shoots for a fuller poinsettia.

Controlling Light Exposure:The amount of light a poinsettia receives controls its ability to produce colored bracts. Growers manipulate this natural process, called photoperiodism, to create colored leaves in time for the Christmas season, by giving the plants at least 12 hours of darkness for eight to 11 weeks.

Thanks, internets. I love when you prove me wrong get me the information I’m seeking!

And our Poinsettia is still hanging in there, although we’ll probably toss it out when it comes time to torture the poor potted Easter Lily that will show up in a couple of months.

The Poinsettia Mystery