A close up of the golden tower spurge


A Close-up View of the

'Golden Tower' Spurge

Euphorbia cornigera 'Goldener Turm'

by Brian Johnston   (Canada)

Spurges, members of the Euphorbiaceae family, display an unusual trait;  they have ‘naked flowers’ which are missing petals, but which possess a whorl of (sometimes) colourful bracts, (modified leaves).

Euphorbias are extremely varied; they can be small and grass-like, or huge trees. The Euphorbia family 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!

The particular hybrid studied in this article is the Golden Tower Spurge, Euphorbia cornigera ‘Goldener Turm’.  Sometimes called the Himalayan Spurge, it possesses green leaves with a distinctive white central vein, and numerous bright yellow-green flower-heads.  (The plant’s developer describes the colour of the blooms as chartreuse-yellow.)  This colour scheme can be seen in the first image in the article.

Although the plant’s description refers to its upright habit, the reality is that the stems are extremely straggly, and often bend through extreme angles, resulting in some of the flower-heads being level with the ground.  This posed problems when photographing the plant, and in some cases the plant’s pot had to be supported 20 to 30 centimetres above the table-top, in a horizontal position, in order to obtain the required images.  Mature stems are red-brown in colour, while immature ones are much paler.

The number of oval yellowish-green bracts surrounding the central flowers is usually three, although as we will see later, the number can be two, or even four.  In the image that follows, the actual flowers can be distinguished by their brighter orange-yellow structures.  Second generation flower-heads, with their associated bracts grow from within the main flower-head.

As you can see in the three images that follow, leaves near the tip of the stalk tend to be much lighter green than those further down the stem.  Usually leaves are in alternate positions along the stem, but there are some exceptions (first image).

The following group of images shows the plant’s strikingly coloured flower-heads.  It is not surprising that the cultivar name “Golden Tower” was chosen for the plant.

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 five 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 two images below show an ‘old’ flower in which the stalk holding the ovary and its attached pistil has bent away from vertical by more than ninety degrees.  The male flowers have shriveled and fallen from the flower, leaving behind the ring of involucral glands.

By contrast, most of the cyathia visible in the image that follows are at a very early stage of development.  Here only a swelling at the centre denotes the presence of the ovary, and no pistil or stamens are present.  Even the involucral glands are a green rather than orange colour.

As the cyathium develops, the ovary begins to protrude from the ring of involucral glands, and the three-lobed stigma extends from the top of the ovary.

In the two images below, the stigma, style and bulbous green ovary (seed producing organ) of the female flower have bent away from their vertical position.  New male (staminate) flowers have appeared, 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!

This image shows an intermediate stage, just as the female flower starts to bend away in order to ‘make room’ for the male flowers that are about to appear.

Even before the cyathium finishes blooming, another generation begins its development.  In each of the four images, a light green pointed ‘baby’ cyathium can be seen growing from beneath the ring of involucral glands.  The number of ‘baby’ cyathia is variable; three can be seen in the fourth image.

Images showing these second generation cyathia can be seen below.

When the male staminate flowers first appear, their bi-lobed anthers are covered by pale green membranes.  These membranes eventually decompose to reveal the abundant pollen grains that cover the tips of the lobes.

As time passes, the male flowers begin to turn brown, and fall away from the cyathium.  In the fourth image, the brown remnants of male flowers can be seen within the ring of involucral glands.

Notice the very large number of pollen grains that adhere to each lobe of a male flower’s anther.

To me, the most interesting stage of development is the fully blooming one.

The two images that follow show clearly the bifurcated tips of each stigma lobe, and the supporting style.  Notice the bulbous tip of each lobe, and the bumpy surface of the female flower’s ovary.

Only two male flowers remain in the cyathium shown below.

Eventually, all of the male flowers die and fall away, leaving only their coloured bases.  Notice in most of the images, that the bumpy ovary is divided into distinct wedge shaped segments.

In the final stage, even the female flower turns brown and falls away, leaving only the bases of the male flowers, and the ring of involucral glands.

Shortly after photographing the Golden Tower Spurge, my local greenhouse brought in another interesting hybrid spurge called Kalipso.  It had the scientific name Euphorbia hybrida ‘Imprkalip’.  The image that follows shows its appearance.  Notice that this cultivar has only four involucral glands, and that the glands are bright red in colour and have light green ‘tips’ that extend from the outer corners.

Compared with the earlier hybrid, this one is smaller, and more compact, and has many more leaves.

Opening leaves tend to be lighter in colour, and have hints of red colouration.

Additional images showing female flowers and immature cyathia can be seen below.

As before, a female flower possesses a pistil consisting of a style which supports a stigma with three bifurcated tips.

The most obvious structural difference is the number, shape and colour of the involucral glands.  In the last couple of images, the sticky coating of nectar can be seen glistening on the surface of involucral lobes.

Unlike the other spurge hybrids that I have studied, this one never develops male flowers.  It is produced by asexual reproduction involving terminal vegetative cutting.

Spurges are popular garden plants, but not for the typical reasons.  Here it is primarily their interesting shapes and subtle colour variations that whet the gardener’s appetite.

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.

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.

 All comments to the author Brian Johnston are welcomed.

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