Plant of the Week, 21st November 2022 – Sun Spurge (Euphorbia helioscopia)

Sun Spurge, Euphorbia helioscopia, is an annual ‘weed’ of gardens and farmland. It looks rather ‘other worldish’, yet it is a European species first recorded in Britain in 1562. Its structure is unusual, appearing like an umbrella, with a straight slender stalk and a flat-topped canopy made of yellowish inflorescences. It stands erect and often alone, 10-50 cm tall; unlike many weeds, it seems to respect geometric principles!

Two views of the Sun Spurge, a species pollinated by flies and dispersed by ants: Left, pollinator’s view from above; right, ant’s view from below. Note: five ‘spokes’ radiate from the central stalk. Each spoke may be divided near the top (to a variable extent). Sometimes one of the five spokes aborts leaving only four. Photos: John Grace.

It has a close relative, Petty Spurge, Euphorbia peplus. This one is less handsome – it lacks the umbrella form and usually it branches in a somewhat untidy way. The best way to tell them apart is that the leaf-margins of E. helioscopia are very finely serrated whilst those of E. peplus are plain. They are both common in gardens, by pavements and roadsides. They also grow in my greenhouse.

The closely-related Petty Spurge Euphorbia peplus, pictured here at the edge of an Edinburgh street next to Polypogon viridis (Water Bent Grass). Both flourish in the detritus that accumulates when streets are unswept. Photo: John Grace.

They belong to the family Euphorbiaceae. It’s a very ‘modern’ family. Flowering plant families emerged in the Cretaceous (145 to 66 million years ago) but the Euphorbiaceae appeared just 40 million years ago and diversified during the Miocene (5-23 million years ago) to produce a bewildering array of trees, shrubs and herbs, some of them with photosynthetic systems like those in tropical grasses or desert cacti (but E. helioscopia isn’t one of these special ones).

Painting of E. helioscopia from Johann Georg Sturm (Painter: probably his son, Jacob Sturm), Public domain, via Wikimedia Commons; first published 1796 in Deutschlands Flora in Abbildungen (‘German Flora in Pictures’). Note the detail of female and male flowers in the bottom-left corner, label b.

In Euphorbiaceae, what looks like the flower isn’t really a flower at all.  It’s more interesting than that. The real flowers are tiny and sit on a leafy platform or cup (John Gerard, in his 1597 Herbal, endearingly calls it a ‘little platter’). These flowers have no petals or sepals. The female is a single pistil and each male flower is just one stamen. There are also nectar secreting surfaces (‘glands’) to attract pollinating insects. The whole structure is called a syathium. Sturm’s painting (part b, bottom left) shows the arrangement better than any photograph. He has drawn five male flowers and one female. The female flower has a three-chambered ovary which swells considerably after fertilization and produces three seeds. The anthers of the male flowers break open when ready and the released yellow pollen sticks to any visiting insect. It might be a fly or bug attracted by smell and the offer of a good landing platform. When it brushes against the pistil, fertilization occurs and shortly afterwards the seeds are formed.  

This image shows the reproductive structures in detail. The globular structures on short stalks are the female flowers each with three stigmata. The yellow dots are the anthers of the male flowers. The four glistening pools are produced on the surface of the nectar-producing glands. The structure and its leafy cup is called the ’syathium’. Photo: ©Chris Jeffree.

In Sun Spurge the seeds are released explosively and (I have read) you can even hear the ‘pop’. I must listen out for this. The seeds have an external fleshy body  (‘elaiosome’) attractive to ants; thus the seeds are carried away to new sites.

The diversity of the family is remarkable. Some examples are shown here. Who would have thought that the rubber tree. the Christmas pot-plant Poinsettia, Cassava as well as various succulents that look like cacti are in the same family as Sun Spurge? Yet they are. The basic flower structure described above has been morphed and coloured during 40 million years of evolution to produce around 7000 species world-wide, many of them useful gifts to humankind.

In the British Isles there are 22 species of Euphorbia but more if you count the occasional escapees of ornamental forms from gardens (Stace 2019). Sun Spurge was formerly considered to be a native, but nowadays it is deemed to be an archaeophyte (i.e. a species that was brought here before AD 1500). The Petty Spurge is also an archaeophyte; perhaps both species crossed the English Channel with invading armies. Both species now occur on all continents except Antarctica.

All members of the family produce milky latex when injured. Sun Spurge’s name in North America is Madwoman’s Milk, presumably alluding to the latex. It isn’t a milk you would ever want to drink. It causes intense skin irritation, and it is highly poisonous. Such strong chemical defences keep herbivores away, and the sap contains compounds that inhibit germination of other species nearby.

Cutting the leaf results in the immediate release of the milky latex. It is a strong irritant and contact with human skin should be avoided. For the chemistry of latex, see Benjamaa et al. (2022). Photo: John Grace.

I found much recent research on Sun Spurge, almost all of it about the chemical composition of the plant. Like most euphorbias this species contains hydrocarbons, especially di-terpenes. One member of the family, Jatropha curcas, was twenty years ago hailed as a future source of diesel fuel. However, hopes that this little shrub was the ‘wonder plant’ to provide hydrocarbons to replace fossil fuels faded as yields were low. But the seeds do contain about 30 % of biodiesel which can be extracted by simply pressing, and this plant may yet be a boon for remote communities.

Seed of the closely-related Euphorbia lathyris L. (Caper spurge) with prominent elaiosome, much-loved by ants. Attribution: Hans Stuessi, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons.

Since ancient times, Euphorbias have been among the most used medicinal plants (Ernst et al 2015). Sun Spurge itself has jatrophane and other similar compounds, and has been shown to have useful biological activity: anti-inflammation, antiviral and anti-toxicity at the cell level (Zhao et al 2022).

The plant is mentioned in herbals, going back to the ancient Greeks and Romans. In Britain, Gerard’s herbal recognised 23 types of euphorbias but his accounts are somewhat confused. Whilst recognising their danger he does say they “cureth all roughness of the skin, manginess, leprosy, scurf, and running scabs, and the white scurf of the head. It taketh away all manner of warts, knobs, and the hard callousness of pustules, hot swellings and carbuncles”. Gerard mentions in passing that they kill fish.

Distribution of Sun Spurge, Euphorbia helioscopia. From BSBI.

Euphorbias feature in Chinese medicine even today; dried material can be purchased from suppliers such as americandragon.com. However, it is not always evident which species is being referred to. Detail is ‘lost in translation’.

In Britain, application of the irritating latex was used to get rid of warts, and poachers are said to have employed this plant to kill fish. There are many folk tales of use and misuse, some of them highly fanciful including the oft-repeated one from the Isle of Man about its use as an aphrodisiac. But, dear reader, I will not tell that story for fear of being the cause of injury in your home.

References

Benjamaa R et al (2022) Frontiers of Plant Science https://doi.org/10.3389/fpls.2022.1008881

Stace C (2019) New Flora of the British Isles. 4th edition. C&M Floristics.

Ernst M et al (2015) Global medicinal uses of Euphorbia L. (Euphorbiaceae) Journal of Ethnopharmacology 176, 90-101.

Zhao H et al. (2022) Phytochemical and pharmacological review of diterpenoids from the genus Euphorbia L. (2012-2021). Journal of Ethnopharmacology 298, 115574. doi: 10.1016/j.jep.2022.115574.

©John Grace