Close-up View of the
by Brian Johnston (Canada)
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
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 17th 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
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
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
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
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
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
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.
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
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
The photomicrographs were taken
using a Leitz SM-Pol microscope (using a dark ground condenser), and
the Coolpix 4500.
A Flower Garden of
A complete graphical index of all
of my flower articles can be found here.
The Colourful World of
A complete graphical index of all
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
December 2008 edition of Micscape.
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