Close-up View of Wood Spurge

Most flowering plants catch your attention with their colourful display of strikingly beautiful blooms. The wood spurge is not one of these! What makes this plant special, is the alien strangeness of its flowers, and the atypical reproductive structures that slowly emerge over the blooming period. In fact, when I first saw the plant at my local garden centre in early spring, I wasn’t sure whether it had flowers at all! Acting on a whim, I brought the most promising specimen home to investigate, and photograph. This article is the result of a week-long adventure in learning about this very unusual species. The wood spurge is indeed “a puzzle, wrapped in an enigma, shrouded in mystery”!

Euphorbias are extremely varied; they can be small, and grass-like, or huge trees. The Euphorbia family (Euphorbiaceae) has approximately 300 genera, and 5000 species. Genus Euphorbia alone contains about 2000 species. Some of the plants are of economic importance. The manioc, caster bean, and para rubber tree are examples. Some, like the poinsettia (Euphorbia pulcherrima), the crown of thorns (Euphorbia millii hybrids), and the African milk tree (Euphorbia trigona), are grown as ornamental plants.

One important characteristic of euphorbias that should be kept in mind, is that they exude a milky sap, called a phytotoxin from cut leaves and stems, that is poisonous, and corrosive to skin and eyes. The phytotoxins produced by euphorbias contain diterpene esters, alkaloids, glycosides and ricin-type poisons. The severity of the symptoms depends on the particular species being handled. Euphorbia, the genus name, comes from a Greek surgeon named Euphorbus, who, it is said, used the milky sap of the plants in his curative potions!

Efanthia wood spurge, the subject of this article, was developed in Germany by the InnovaPlant company, and is a hybrid cross of Euphorbia amygdaloides and Euphorbia martinii. It has deep green leaves, and thirty to forty centimetre long stems that take on a deep purple colour in cool weather. During the spring, each stem is topped by yellow-green flower-heads.

When I first obtained the plant, it had the appearance shown below. The flower-like structures in the images are collections of bracts (modified leaves). No actual flowers are in view at this point.

A closer view of the front and back of one of the collections of bracts, shows that some still have purple colouration. The outside temperatures in the area where the plants were growing hovered about 5 degrees Celsius overnight, and this low temperature environment maintained the plant’s purple colouration.

The intensity of purple in the bracts is variable. Notice in the image on the right, the strange red knobby protuberance at the base of the short stalk that connects a bract to the main stem. Stems, and the undersides of bracts, are hairy.

Under the microscope, the edge of a bract can be seen to be composed of cells with a deep red colour. Numerous red spots are also present on some bracts.

A higher magnification reveals the individual cells that produce the colour, and the bases of the hairs that grow from the surface. In the image on the right, oval structures called stoma and guard cells, which control gas entry into the bract, are visible.

What you are looking at below is a single flower-head composed of many flower-heads each of which contains individual flowers. Although this may seem to be an absurd statement - it is not. All will be revealed later!

The three images that follow show the wood spurge’s leaves, which tend to form a loosely pine-cone shaped cluster midway up the stem. (The many white water-spot marks on the leaves are due to frequent watering using the very ‘hard’ [calcium containing] water in my area.)

The reverse surfaces of the leaves are covered in soft-to-the-touch, fine, downy hairs.

About a day after bringing the plant indoors, where the ambient temperature was warmer, changes began to appear in the flower-like structures.

The bud-shaped objects began to move away from the double rings of yellowish-green bracts, as their stalks lengthened.

At the point of connection of the stalks to the top of the stem, a double-lobed, bright yellow structure became visible.

As the stalks grew in length, the structure that “looked” like a flower at an earlier stage, began to look less like one.

Eventually, after another day had passed, the spherical bud-like structures began to open, and show their strange contents.

All of these structures contain four horseshoe-shaped objects, but only some possess the bifurcated three-lobed structure seen in the images that follow. Stranger still, the horseshoe shaped objects had a dry upper surface, unless a three-lobed structure was present, in which case droplets of liquid were present! I repeat - “a puzzle, wrapped in an enigma, shrouded in mystery”

Finally, some answers to earlier questions. Spurges possess a highly specialized inflorescence (bloom), called the cyathium. This cyathium is a cup-like cluster or whorl of bracts that encloses a group of structures including a single female (pistillate) flower, ringed by several male (staminate) flowers. Both the female and male flowers are enclosed within a ring formed by four horseshoe shaped glands (involucral glands). These glands secrete a shallow layer of nectar which is attractive to flies, and other insects. (The name cyathium is from the Latin kuathion, diminutive of kuathos, meaning “ladle”.)

The image below shows a single cyathium, with nectar coated involucral glands, and central bifurcated three-lobed stigma (female pollen accepting organ). Note that the stigma and supporting style form a single flower. (The involucral glands and bracts are part of the cyathium, but they are not part of the flower!) Where are the male flowers that should circle the female flower? They haven’t appeared yet! (One must be patient when studying the wood spurge!)

A single lobe of the three-lobed stigma can be seen in the photomicrograph that follows.

Higher magnification photomicrographs reveal the cellular structure of the bifurcation point, arm, and tip of the stigma.

At the point where the style supporting the stigma passes through the gap between the four nectar producing glands, there are many long hairs.

Strangely, the occasional cyathium does not possess female (or male) flowers. Only the involucral glands are present. Stranger still, the glands do not produce noticeable nectar in this situation! In some cases, the female flower eventually appears, but in many others, the cyathium appears to be infertile.

The cellular structure of different sections of a gland can be seen in the photomicrographs that follow.

Several late developing, or infertile flower-heads can be seen below.

At this developmental stage, two additional structures within the cyathium become large enough to distinguish. Their function (at this stage), is still a mystery.

When one of these structures seen in the image at left, is examined under the microscope, it appears to be composed of two overlapping bracts. Can you guess what it is? More later.

If we turn our attention again to a fertile cyathium, several days have passed, and there are more changes. New flowers have appeared, and the single female flower has disappeared - well, not really- the stigma, style and bulbous green ovary (seed producing organ) have bent away from their previous position! The new flowers are individual male (staminate) ones, with bright yellow pollen on their bi-lobed tips. Having a flower-head’s pistillate flower appear much earlier than its staminate flowers tends to decrease the chance of self-fertilization. The stigma can receive insect carried pollen from other plants before pollen grains from the flower-head’s nearby male flowers become available. By bending the female flower away from nearby male flowers, the possibility is reduced even more!

Closer views of the staminate flowers can be seen below. Notice in the right-hand image, that each pale green anther has two lobes, and that it is supported by a white filament that has a larger diameter base.

A single anther, and its supporting filament, can be seen below. The grooved, ellipsoidal pollen grains that coat the anther lobes can be seen in the higher magnification image on the right.

Phase-contrast illumination provides a much clearer view of a pollen grain’s structure.

The cup-shaped structure that holds the base of the filament can be seen in the image below. The upper portion of the filament was removed to take an earlier photomicrograph.

Again, several days have passed, and more changes have occurred. The two mystery objects mentioned earlier have grown in size, and their stalks have lengthened. Have you guessed that each is an immature cyathium? The entire process is starting again! What an unusual plant!

The image that follows shows particularly well, the new position taken by the pistil (style, ovary and stigma).

By removing the two bracts that form the cyathium’s cup, its contents are more easily seen. The female flower has bent over and down, in the direction of the viewer. The male flowers are still in their vertical positions, and the immature cyathia are attached by their stalks to the flower-head’s stalk.

If you compare the image below, of a blooming flower-head, with the third image in the article, you can see how dramatically the flower-head has changed over a period of a week.

Although I admire colourful, simply structured flowers as much as the average person, I must admit to a bias towards plants with an unusual or complex structure. As a macro-photographer, their intricate flowers provide more of a challenge! The wood spurge is a perfect example. There have been few flowers that have provided the same level of fun in the learning, and picture taking processes as this one did!

Photographic Equipment

The macro-photographs were taken with an eight megapixel Canon 20D DSLR equipped with a Canon EF 100 mm f 2.8 Macro lens which focuses to 1:1. A Canon 250D achromatic close-up lens was used to obtain higher magnifications in several images.

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

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© Microscopy UK or their contributors.

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