A Close-up View of the Corn Cob Euphorbia


A Close-up View of the

Corn Cob Euphorbia

Euphorbia mammillaris

by Brian Johnston   (Canada)

This plant can be summed up in a single word – peculiar.  If it were not in bloom, you might guess that it is a cactus.  It does have tubercles, (bumps or swellings), aligned in vertical rows like an ear of corn, and what look like spines.  However, cacti have clear sap, while this plant exudes a milky liquid.  Certainly its flowers, seen in the image above, are not at all like those of most cacti.  What could it be?  The answer of course, is an Euphorbia.

The genus Euphorbia is enormous, with over 2000 species displaying astonishing variations.  For example this plant and the Poinsettia are both Euphorbia.  Is that different enough?  The genus name refers to Euphorbus, Greek physician to Juba II, the King of Mauritania.  Mammillaris, the species name is translated as having nipples or breasts, and this refers to the many swellings on the surface of the club-shaped plant.

Near the top of the plant, there are whorls of tiny leaf-like structures above the ring of blooming flowers.  Beneath the flowers are a few of these thick leaflets, and some that have turned a bright red colour.  Also near the top of the plant are thin pink rod-like structures which eventually dry out to form stiff brown cactus-like spines, although they are not sharply pointed.

The four images that follow show views of the top of the plant.  At its tip, there are several whorls of green leaflets.  Further down the column, these leaflets grow from the centre of each tubercle, and many are bright red in colour.  Still further down, these leaflets have dried up, and fallen from the plant.  Some, but not all tubercles have a fleshy pink, rod-like structure which is a modified flower spike.  Eventually these spikes lose their colour and dry to form grayish-brown woody spines.

The relative positions of the small leaflets and modified flower spikes can be seen in the two images that follow.  The leaflets grow from the exact centre of a tubercle, while the modified flower spikes grow at the intersection point of two tubercles.  The closer view on the right shows the red tops of modified flower spikes as they begin to grow out from between two tubercles.

Spikes do not have a sharply pointed tip, and some have microscopically small leaflets growing from the column near its tip.  Notice how neatly the tubercles fit together over the surface of the column.

A Euphorbia flower, more correctly called a cyathium, is positioned on top a stalk referred to as a pedicel.  There are no petals, although in the images below, one might think that the five rounded orange-yellow structures might be.  These are actually the flower’s nectar producing glands (nectaries).  Protruding from the centre of these glands is the single stigma which consists of three forked tips supported by a stubby style.  Since close examination failed to show any evidence of stamens, I suspect that this species has male flowers on a separate plant.

The two images that follow show a whorl of red-edged greenish leaflet-like structures immediately beneath the nectary glands.  These are petal-like bracts called petaloid appendages.

The structures that make up a Euphorbia mammillaris flower can be seen clearly below.  Growing from the tubercle is the whorl of petaloid appendages, from the middle of which the pedicel emerges.  The pedicel ends in the ring of nectary glands, from which the flower’s style protrudes.  The style branches into three stigma lobes, each being forked.  Newer flowers tend to be yellow, while older ones tend toward orange.

At the centre of the ring of nectary glands there is a flat red area surrounding the style.  This contrasting colour may be an adaptation to help guide insects to the flower.

Closer views of this red area show that it is covered in fine white hairs.  Broader red hairs can be seen in the image on the right, growing up against the style.  Also note that the surface of nectary glands is dimpled.

After a period of time, (approximately a week), the nectary glands begin to disintegrate, leaving only the pistil and petaloid appendages.

Unlike the cactus, whose defensive spines are present for its entire life, the Corn Cob Euphorbia’s spine stand-ins, the modified flower spikes, turn brown and drop off after several months.  The tiny leaflets at the centres of tubercles also fall off leaving distinctively shaped scars.

Both flower-spike and leaflet scars can be seen in the image on the left below.  On the right is the clear evidence that the plant is a Euphorbia, and not a Cactus.

The following group of images show the small leaflets of the plant from a variety of points of view.  Notice that a leaflet has a red, concave upper surface, and a yellow, convex lower surface.  The leaflet’s thickness is also unusual.

Newly emerged modified flower spikes can be seen below.  Each green spike can be seen to have a number of extremely small leaf-like structures near its tip.

At a later stage, the spike has grown longer, and has an increased diameter.  The closer view on the right shows clearly the tiny leaf structures at its tip.

My local greenhouse had a group of Euphorbia mammillaris plants for sale the day that I paid a visit.  Only one was in bloom.  Its alien-looking flowers prompted be to bring it home, and this article is the result.

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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