Euphorbia portlandica is a very small plant, native to parts of Britain and named after the Isle of Portland. For those of us who enjoy hearing the BBC shipping forecast playing in the background, this strange sequence, “Thames, Dover, Wight, Portland, Plymouth.” is very familiar, and gives us a rough idea where Portland is. We’ve also heard of Portland stone, the limestone building material famously used for many important buildings like St. Paul’s Cathedral and the Cenotaph.
Such fame somewhat belies Portland’s small size. Just four miles long by a mile-and-a-half wide, the Isle of Portland juts out into the English Channel to the south of Weymouth with the 18 miles of Chesil Beach stretching away to the west. What a glorious place to call home.
But, of course, it also grows in many other places. It is found along the Atlantic coast from Gibraltar to Scotland and, in Britain, from Hampshire westwards and north as far as Galloway, but is rare and very local. .
Why do I grow Euphorbia portlandica?
Because it chooses to thrive in my garden and I like plants that succeed with little, or no, intervention. In the wild it grows in sandy, dry, coastal locations so has, perhaps, found a home from home in my garden.
I first came across it in one of Margery Fish’s books, which I used to read avidly when I first started gardening. She was a judge of floral exhibits at shows and first noticed it in miniature flower arrangements.
She found out all about it and made a pilgrimage to Portland to collect seed. She wrote, “I am quite certain I shall have Euphorbia portlandica for the rest of my life. It makes neat little bushy plants about 6 in high in very glaucous foliage and keeps its children close at hand.” I thought that was a wonderful way to explain that this little wild flower wouldn’t become a weed. I can’t remember where I bought my plant but its progeny are still with me over twenty years later.
Roger Turner, in his book, Euphorbias A Gardeners’ Guide, describes Euphorbia portlandica as “small, unusual, if rather insignificant-looking.” However, he was setting it alongside many other much larger and more significant euphorbias so, in that context I suppose that description is fair. But if you’re looking for the unusual, something to fill a niche, that won’t take up too much room, that will allow other plants to take the limelight above it, that will get on with life in a classy and unassuming manner, and you have a suitable location, then this little euphorbia fits the bill.
Where does this tiny plant fit in the plant world?
Euphorbia portlandica, the Portland spurge, is one species in a large and very diverse genus that belongs to the family Euphorbiaceae. There are about 2000 accepted species of Euphorbia found in almost every corner of the globe. They range from tiny annual weeds to large, long-lived trees – Euphorbia ampliphylla is a succulent tree from tropical East Africa, which can reach 30m in height. There are just over a hundred species native to Europe and about twenty in Britain, but most Euphorbia species come from tropical and subtropical regions. Euphorbia is characterized by the presence of a unique inflorescence, termed a cyathium, and the production of a white latex, which the plants exude when cut.
The ones we grow in gardens in the UK are usually referred to as herbaceous perennials and originate mainly from Europe or temperate Asia. There are some wonderful plants among this group and lots of interesting information to find but it’s just this one diminutive species that I’d like to tell you about.
What does Euphorbia portlandica look like?
Each Euphorbia portlandica plant has a number of stems arising from the centre. In my plants they are about 15cm long although the books say they can reach 40cm. As you can see in the image, the bases are bare of leaves and red in colour. Further up are the stem leaves, reverse-lanceolate in shape, 10 mm by 5 mm at the widest part.
Then comes a whorl of five whorl leaves, which are oval in shape, 10 mm by 7mm. This marks the beginning of the floral head. Five rays grow from the whorl, each one ending in a structure that is repeated over and over to produce the floral head.
This structure consists of two specialized leaves called cyathophylls, sometimes referred to as floral leaves. These are roughly triangular, about 12 mm across with an obtuse apex with a fine point and form a shallow saucer. In the centre is a cup-like structure called a cyathium, a word derived from kyathos, Greek for a cup or dipper. It consists of an involucre of fused bracts, just 2 mm across, with four yellow, horned, nectar glands on the rim, which encloses one female and, in this species, five male flowers.
Either side of the cyathium are what look like two buds, which, in fact, extend into subsidiary rays, each with two cyathophylls, a cyathium, and two more ‘buds’. This branching of the rays is repeated, so producing the floral head, the cyathophylls getting smaller and the rays shorter each time. Sometimes there is only one branch formed. In my plants, after the initial rays, branching occurs three times. Axillary rays can also be produced directly from the stem below the whorl.
These stem and whorl leaves and the cyathophylls are glaucous and fleshy with a prominent midrib beneath. They contrast in colour with the yellow nectaries and the somewhat brighter green ovary.
Now we need to get in closer on the flowers
In the first two images below I have removed the ‘buds’ to give a clearer view of the cyathium. You can see the involucre and the nectaries on its rim. In the third image you can see the saucer shape formed by the overlapping cyathophylls and the four horned nectaries shiny with nectar.
The flowers emerge from the centre of the cyathium. They are unisexual and have neither sepals or petals. The single female flower has a pedicel and a three-celled ovary, above which there are three styles, each divided to form six stigmas. There are five male flowers each consisting of a pedicel, filament, and single anther, which has two lobes. However, I have only seen one mature male flower at a time. Timothy Walker, in his book Euphorbias, explains that the stamens drop off as soon as the pollen is shed so the number visible at any one time will vary.
The female flower develops before the male ones to reduce the chance of self-pollination. It is upright at first but hangs over the edge of the cyathium as the fruit develops. Flies and ants are thought to be the main pollinators although beetles, wasps, and bees are occasional visitors.
Where does the next generation of Euphorbia portlandica come from?
The fruit is a capsule, which you can see in its early development in the images above. It is distinctly three-lobed with nodules each side of a groove that runs along the crest of each lobe. It matures very quickly, becomes erect again, and dries to a light brown in colour.
The capsule is a schizocarp, which means that it splits into single-seeded parts, in this case three. The outer wall, the pericarp, of each part, then splits along the groove and the seed is expelled. This expulsion is really quite explosive and, even in this very small plant, the seeds travel some distance. The image shows a mature capsule, three seeds, and somes pieces of the pericarp after splitting.
The seeds are about 2 mm long and have a pale-coloured, oil-rich protuberance called a caruncle. This is thought to be attractive to ants, which carry the seeds off and so aid dispersal. If the seeds find suitable conditions germination occurs and the next generation of seedlings is produced.
What was I thinking?
As soon as I started on this project, to look closely at some euphorbias, I realized that, although I have grown a number of different euphorbias for many years, I had never really got to grips with their structure or its nomenclature. A little way into the research I remembered why. It’s confused and confusing.
What’s the problem? Put simply, there are many different names for the same part of the plant, which makes trying to understand the structure and function of the different parts very difficult. Leaves have epithets to describe their location. Stem leaves and whorl leaves seem to be standard but I have seen the latter referred to as bracts of the umbel, umbel leaves, pseudumbel leaves, and ray leaves. The third type can be floral leaves, raylet leaves, bracts, partial bracts, leaves of involucre, cyanthium leaves, or, the one I settled on, cyathophylls.
The floral head can be seen as having the appearance of an umbel. It is not an umbel because that is used to describe an inflorescence. It is sometimes described as a pseudumbel. It’s not a raceme. It could be a cyme but, where a cyme has multiple branches, it’s called a pleiochasium. However, in Euphorbia, the floral head is not an inflorescence so pseudopleiochasium would be more accurate. You get the picture.
I decided that the way forward was to study an actual plant closely. I chose Euphorbia portlandica because, even though it’s small, it has a simple structure. It’s also one of my favourites and I thought others might find it interesting. In order to take photographs that showed all the features I wanted to describe, I had to examine the plant in detail and was struck yet again how much you notice when you look really hard.
I found Timothy Walker and Roger Turner’s books particularly useful and a website about euphorbias was instructive, although it hasn’t been updated since 2012. But it did give me a link to a very interesting study from 2007 on the cyathium in Euphorbia. Right at the end of the paper there are some fascinating scanning electron micrographs showing the deveopment of the cyathium but seeing the coloured one of a cyathium, and comparing it to my photographs, finally made the penny drop!
What have I learned from my delve into euphorbias? I now understand the basic structure of a euphorbia plant. I know that other species and cultivars will differ but I also know what to look for and how to identify the different parts. I might have trouble picking a name but have decided which I think are the most reasonable.
However, the most important feature, and the one I will always look at first from now on, is the cyanthium, the distinct structure found in all 2,000 species of Euphorbia but in no other group of plants.
With my new-found knowledge I’m rather looking forward to next spring so that I can identify the similarities and differences beween Euphorbia portlandica and my other euphorbias. I will be looking at them with fresh eyes.
Find more information
- Plants of the World Online – Euphorbia portlandica
- Comparative ontogeny of the cyathium in Euphorbia (Euphorbiaceae) and its allies
- Euphorbia – Global inventory of the spurges
- Euphorbias RHS Wisley Handbook by Timothy Walker
- Euphorbias A Gardeners’ Guide by Roger Turner
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