Some new spurge discoveries this week: a species new to me, and some insights into where these weird weeds came from. There’s a reconstruction of the supposed ancestral spurge, and a draft family tree, some of whose branches have criss-crossed the Atlantic. It may be long, but these plants tell an amazing story about our planet.
The latest spurge I found,on Boxing Day, was growing on a sunny bank near the Etang de Thau, and fittingly it’s called Euphorbia helioscopa [sun-watcher], sun spurge in English, Euphorbe réveil-matin, or Petite éclaire in French. All because the umbels open and face the sun in the morning.
The characteristics by which you can identify it include these: the leaves are oval, wider above the middle and finely toothed, red-tinged as is the stem; it has a flat terminal inflorescence consisting of a five-rayed umbel, and kidney-shaped nectar glands.
Another discovery this week was the website of an Italian naturalist, Dr Giuseppe Mazza, who describes in a fine style the way the capsules of this species split open explosively, like most Euphorbias, to disperse the seeds: ‘And during a quiet autumn afternoon, between the herbs of the orchard, we can easily hear, as well as see, the babies of the Euphorbia helioscopa leaving for conquering the world.’
I do recommend a visit to his site, photomazza (click here). In a long translation of his article on Euphorbias he says the flowers are heirs ‘of the ancient bisexual corollas of a wicked botanic ‘Dynasty’…where poison, castration and nudism are of the family’. Doesn’t sound like the textbook I had for A level.
Now for evolution, and there are a couple of important bits of background. First, understanding something of the movement of the continents on their tectonic plates is vital to understanding how older plant forms have evolved: plant communities get separated and evolve in different directions, and get carried into new climate zones which exert new selections of the fittest. But it is like trying to understand three-dimensional chess. Try this video to get a general idea – I think it’s stunning, 600 million years in 90 seconds:
There’s also a good sequence of maps here.
Most diversity in the Euphorbiaceae family is in southern and eastern Africa and Madagascar, and most species are in tropical or subtropical Africa and South America. In general, greater diversity indicates greater age of a group of plants. This suggests a possible origin for the family in the South Africa part of the supercontinent Pangea (Gondwana in the map below), and a spread to the part which would become South America.
Pangea 152 million years ago
Pangea – shown above when there were just ferns and pines,no flowers – had finally broken up by about 100 million years ago, when South America split off from Africa and the South Atlantic began to widen (below). This theory of the origin of these plants is borne out by molecular studies, and the few fossils we have, which give an estimated age for the Euphorbiaceae family of 114 million years – in other words, very, very old : flowering plants only began to evolve about 140 million years ago.
Pangea 94 million years ago
(Colour maps from scotese.com)
Now the second bit of background. The study of evolution has been revolutionised in the last few years by molecular genetics – the analysis of plant DNA to find particular genes or markers, and then comparing several species to see which has what. The more genes in common, the more close the relationship, and a gene common to all suggests an ancestral feature. In this way a family tree can be drafted, and even dated – because we can estimate the rate of change in genes through mutation. I found a recent (2012) molecular study of Euphorbia, and if you’re not put off by a bit of jargon, it’s worth a look for the pictures and diagrams, though the middle section gets way too technical for me (you can find it here) . Scientific papers are a bit like gunpowder tea: dry and shrivelled at first sight, but the longer you let them soak in your mind, the more flavours they release. Basically, from looking at the distribution of 10 genetic markers over 176 species, this is what the researchers come up with:
The Euphorbia genus can be divided into four big groups, or clades in botan-ish. One of these, called the Esula clade, contains 500 species found in temperate regions of the northern hemisphere, from the eastern Mediterranean to central Asia . This group are almost all herbaceous, with a straight, leafy unbranched shoot and a terminal inflorescence. It contains all the wild species I see here, and the other European species.
Let’s take a break to look at one of these: Euphorbia serrata. It has leaves with finely toothed margins, and which become broader higher up the stem. The umbels have 3-5 rays, and the bright yellow bracts are wide – the inflorescences make quite an impact.
Euphorbia serrata in May
The other groups seem to have developed successively in evolution from this stock, and comprise a group of 200 xerophytes (dry-adapted species) in South Africa; a group of 700 different species in Africa and Madagascar; and a group with half a dozen African members and about 600 species in the New World (including Poinsettias). The similarities in the last group suggest an early African origin and some evolution there, followed by ‘a single dispersal event’ to the new climates of the New World, where variation ran wild. Now it seems that the Euphorbia genus evolved about 42.5 million years ago, long after South America had sailed away on its tectonic plate. So how did Euphorbia cross the ocean?
We don’t really know. To get there like humans did, through Asia and over the Bering Straits to Alaska, would have involved crossing inhospitably cold climate zones. Humans could dress up warm, plants can’t. So it has been suggested that there was a land bridge between northern Scotland and Greenland, and then to Baffin Island, about 40 million years ago when these regions were much warmer than they are now. Also, they were then, and are now, much further south than Alaska. Other genera in Euphorbiaceae, such as Croton, had evolved in America from the original 100 million-plus year old stock, but seem to have spread eastwards to Europe at about the same time. The land bridge had gone by 35 million years ago. Could plants travel so far in the time available? Think of it: if seeds disperse a metre a year, that’s a kilometre in 1,000 years, 1,000km in a million. One thing geological time has got plenty of is time.
Now for the reconstructed ancestor-spurge of around 42 million years ago in Africa: it turns out that these most resembled the Esula clade, in other words the kinds of Euphorbia I’ve been featuring. It was a non-succulent, woody plant with leaves arranged in a spiral around the stem, and cyathia in terminal inflorescences. The basic plan was probably for five of each flower part – now reduced to three compartments in the ovary of the female flower, a single stamen in the male flower, and commonly four nectar glands. The family forebears before this were probably wind-pollinated, with separate groups of male and female flowers on each plant: as Euphorbia evolved from this the flowers moved closer and the bracts became petal-like, and nectar glands developed, to form the cyathia (unique to Euphorbia) which could attract insects – these weren’t around when the Euphorbiaceae family started over 100 million years ago.
Molecular genetic studies show a slow and steady rate of evolution in the Esula clade, as species arose to adapt from the subtropics to the increasing extent of areas in the northern hemisphere with temperate conditions,including cold and dry seasons. The poisonous milky sap is an adaptation to deter herbivores which were common in temperate grasslands.
Further, these genetic studies show that xeromorphic features in the genus as a whole, such as spines and succulence, have evolved on at least fourteen separate occasions over the last 40 million years: an outstanding example of parallel evolution.
Well done for getting this far. A final photo call for Euphorbia cyparissias, or cypress spurge, so named for the many stems whose slender leaves look like young pine shoots – the photo was taken back in May last year.
Euphorbia cyparissias in May
Euphorbia is such a rich genus, and carries the traces of 40 million years of the history of our planet, its continents and its climate. The more we study it, the more it has to tell us.
I’ve been trying in the music selections for these last three posts to mirror the way these species have travelled out of Africa, and through Europe to America, and back again, sometimes coming up with the same innovations in different places. In the first was Sun Ra, who claimed to have been born on Saturn, but in fact came from Birmingham, Alabama, and who used imagery and costumes from Egypt. Second, Randy Weston who came from New York, but lived and worked for many years in Morocco. Today’s choice is the Ethiopian Mulatu Astatke, who was educated and began his career in Britain and then New York, before returning to his native country. He’s made some wonderful recordings in the last few years, and from the highly recommended album Mulatu Steps Ahead (2010) here’s a live version of an appropriate title: ‘Motherland’. On trumpet is the American-born British resident Byron Wallen. You can’t pin a good musician (or plant) down.
Coming up next: the ‘Kewdunnit’. More of a ‘whydunnit’, as you’ll see.