A close up view of the Poinsettia

Like the Christmas tree, the brilliantly coloured Poinsettia plants which seem ubiquitous during early December, are a certain sign that the holiday season is near.

In its native habitat, (the tropical areas of Central America and Mexico), the Poinsettia has an open, rather weedy habit, with a height of from 0.6 to 4 metres. For this reason, the production of compact potted plants with many flower-heads was difficult – so difficult in fact that only one greenhouse owned by the Paul Ecke family of Encinitas California found the secret. For many years they had a virtual monopoly, and sold their spectacular botanical marvels throughout the world. In the 1990’s however, a university researcher discovered the secret, and published the production details. Since then the technique has been used worldwide by other greenhouses to grow Poinsettias for the Christmas season.

The genus Euphorbia is famous for the astonishing variations in structure of its diverse species. Tiny spurges, and giant, cactus-like succulents form the end points in the size scale. Positioned somewhere in the middle, Euphorbia pulcherrima, or Poinsettia as it is more commonly known, is the most popular potted plant in the United States. In the year 2004, approximately 61 million plants were sold! This is even more remarkable when one considers that most Poinsettias are sold in a single six week period before the Christmas holiday!

Dr. Joel Roberts Poinsett, the first United States Ambassador to Mexico in the 1820’s discovered the beautiful shrub in the countryside, and took cuttings back to his greenhouse in South Carolina. Because of the plant’s extreme popularity, William Prescott, a historian and horticulturist, was asked to give Euphorbia pulcherrima a new name. He called it Poinsettia, after its discoverer. The plant’s species name was assigned in the 17^th century by the botanist Juan Balme, who chose pulcherrima – translated as ‘most beautiful’.

In this article, I have chosen to study two Poinsettia colour variations. The first, a relatively recent cultivar, has a pink and white colouration, while the second possesses the more “normal” bright red hue. This second plant will be examined in greater detail in order to reveal the details of its reproductive structures.

Although the pink with white edged structures in the images appear to be the flower’s petals, this is a misapprehension. In fact, the flowers (in bud stage) are actually the tiny green spherical objects at the base of the faux-petals. The petals are not petals at all, but instead are bracts – modified leaves.

In this highly decorative cultivar, the bracts are much more three-dimensional than those in the more popular red form. This highlights the vein pattern, and adds greatly to their visual appeal.

It appears that the chemical signals that trigger the formation of a pink bract are weaker in the occasional leaf, as can be seen near the top centre of the image below. In this bract, the colouration is intermediate between a normal green leaf, and a full pink bract.

To me, these “almost” bracts are particularly striking because of the increased visual contrast between their greenish background, and irregular pink spots.

Higher magnifications reveal the complex vein structure in the lower section of an “almost” bract.

Although this vein structure is clearly visible on the top surface of a bract, it is even more three-dimensional on its lower surface. Notice as well, that the pink colouration is less saturated on its lower surface.

By altering the angle of the illuminating light source, the bract’s raised veins can be made to cast shadows, thus producing an enhanced three-dimensional effect.

The images that follow show the two Poinsettia cultivars photographed for this article. The one on the right is the common form used as Christmas decoration, and it will be examined in detail in the rest of the discussion. I personally prefer the pink cultivar, not only because of its novelty, but if the truth be told, because it is easier to photograph. (One of the difficulties in digital photography is the tendency of the camera’s sensor to clip the red channel, resulting in loss of detail in red areas. This can be partially overcome by decreasing the exposure, and by careful compensation in post-processing with Adobe Photoshop. The many additional steps involved adds considerable time to the overall process however.)

As you can see below, the top surfaces of the red cultivar’s bracts are monochromatic. There are also more of these bracts around each flower-head than are typically seen in the pink cultivar.

The undersides of the bracts have contrasting white veins. Beneath the flower-head, the leaves and bracts have an alternate arrangement on the stem, while in the flower-head, multiple bracts are attached in rings to its top-most section.

Deep within the flower-head, a tiny red proto-bract can be seen developing immediately beneath one of the cup-shaped flower holders.

This proto-bract can be seen more clearly in the higher magnification images that follow. Notice the large diameter of its main vein compared to the rest of the structure.

If one of the plant’s stems is cut through, it is evident that it is more of a tube, than a rod. This tubular stem results in a very strong, sturdy support for the overhanging upper portions of the plant. (The difference in stem colour evident in the two images is due to different lighting conditions.)

As was the case in the earlier cultivar, “almost” bracts occur here. For some reason their development into true bracts is aborted, and they remain at this intermediate stage for the life of the plant.

Some, like the one shown on the left below, are more leaf than bract, while others, like the one on the right, are more bract than leaf.

If the underside of one of the more bract-like structures is examined at higher magnification, shadow-like green areas are clearly visible along the veins.

Near the main vein, the green areas are larger in size. Notice in the image, that although the main veins are white to pink in colour, the fine veins visible in the upper left of the image have a reddish hue.

The photomicrographs that follow show some of this very fine vein structure. Notice in the higher magnification image on the right, that fine hairs grow from the veins in areas where branching occurs. Also note that the base of each hair is bright white in colour.

It appears that the underlying structure of a vein is white, while the top cellular structure imparts its reddish-pink colouration.

The image below allows a comparison of the underside colour of true leaves, intermediate bracts (lower right), and true bracts. It’s interesting to note that the stalks of green leaves are red in colour, while the stalks of red bracts are pale pink in colour. For some reason, maximum contrast seems to be desirable!

Notice the strange curvature in the secondary veins as they reach the edge of the leaf. Although the leaf itself is not scalloped, the vein structure near its edge appears to be!

At higher magnification this scalloped pattern is less evident.

At the centre of the colourful ring of bracts is a cluster of green and yellow structures called cyathia. Each cyathium is an inflorescence, or flower cluster, comprised of many individual male flowers, and a single female flower. The ones shown in the images below are in full bloom, as the tiny yellow dots closest to the viewer indicate the presence of pollen. A single, very distinctive mouth-shaped structure occurs on the side of each cyathium. This is a nectar producing gland that attracts insects to the flower-head.

If the bracts are removed from beneath the cyathia, the Poinsettia’s reproductive structures become easier to see. Each cup-shaped cyathium has a red, fringed top, and contains many male stamens, each of which is a single flower. Hidden from view, deep within the cyathium is a single, very small female flower composed of a pistil and ovary. Note that neither male nor female flowers possess petals. Like most other members of the Euphorbia genus, the Poinsettia has a sticky, milky, latex-like sap that is exuded whenever the plant’s stems or stalks are damaged or severed. However, unlike most other Euphorbia species, this one’s sap is not poisonous. (This contradicts a persistent “old wives’ tale” that states that humans and their pets should avoid contact with, or ingestion of, the sap.) What is true is that sensitive individuals may develop skin irritations when coming into contact with the liquid.

Unopened buds on the ends of short stalks can be seen below. At the back of each immature cyathium, the upper surface of the nectar gland is just visible.

Occasionally, for some unknown reason, one of the male flowers forces its way through the opening at the top of a cyathium bud. In the images below, a precocious male flower has even begun to produce pollen!

In the two views that follow, many male flowers can be seen extending out of cyathia. Each flower consists of a bright red filament which supports a bi-lobed anther. Each anther lobe is encrusted with bright yellow pollen grains. (When purchasing a Poinsettia plant during the holiday season, avoid ones that have visible pollen grains! Such plants are at a late stage of development, and will soon drop their colourful bracts. Choose instead, a plant with cyathia in bud stage; this will increase its display life.)

The two images below show the mouth-like nectar gland growing from the side of each cyathium. As the gland develops, its colour changes from green to bright yellow.

Male flowers in a cyathium do not develop at the same rate. A group of immature, orange-yellow anthers can be seen below. At this stage they have not yet released their pollen.

Higher magnification views of these immature anthers can be seen below. Notice that each anther lobe has a distinctive groove on its surface. Also note that the two lobes are connected to a broad, bright red filament, and droop, appearing to rest on the top of the filament.

This can be seen more clearly in the image that follows. On the left is a mature male flower with its red filament topped by the pollen covered, bi-lobed anther. On the right, an immature male flower with its two pollen-free lobes hanging like floppy ears, is visible.

A mature male flower produces copious amounts of pollen. The grains are released from the upper portions of the two anther lobes. In many cases, they pile up to a considerable height, as can be seen in the image on the right below. Although the filaments of many flowers are thin and fragile, here they are exceptionally sturdy.

As the male flowers (stamens) age, their filaments deflate, and take on a shriveled appearance. This can be seen in the second image below, which shows a photomicrograph of the upper portion of a stamen.

Higher magnification photomicrographs reveal the dimpled surface of an ellipsoidal pollen grain. A longitudinal groove bisects the grain’s surface.

Pollen grains adhere not only to the anther, but also to its supporting filament.

Although the most popular common name of Euphorbia pulcherrima is Poinsettia, there are alternatives. Mexican flame leaf, Christmas star, and Winter rose are several examples. The Aztecs referred to the plant as Cuitlaxochitl, which means “Star flower”. They used the plant to produce a red dye, and to prepare a medication which supposedly treated high fevers. Whatever the name, this plant must rank amongst the most popular in the history of mass-market cultivated flowers. Its widespread appeal is completely understandable!

Further Information

The Poinsettia Pages http://www.urbanext.uiuc.edu/poinsettia/

Photographic Equipment

The low magnification, (to 1:1), macro-photographs were taken using a 13 megapixel Canon 5D full frame DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.

An 10 megapixel Canon 40D DSLR, equipped with a specialized high magnification (1x to 5x) Canon macro lens, the MP-E 65 mm 1:2.8, was used to take the remainder of the images.

The photomicrographs were taken using a Leitz SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.

A Flower Garden of Macroscopic Delights

A complete graphical index of all of my flower articles can be found here.

The Colourful World of Chemical Crystals

A complete graphical index of all of my crystal articles can be found here.

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