Sun Spurge Euphorbia heliscopia

I am an amateur naturalist trying to discover everything living in my garden.

After a series of postings on some of the smaller and arguably more curious lifeforms present in my garden, today's posting features something I suspect most of you will find all too familiar: a garden weed.

Growing in my vegetable patch (at (0.1,1.7) - see here) is the weed seen in photo 1 (click photos to enlarge). It has quite happily survived the British winter frosts.

Working with my copy of The Wild Flower Key (F. Rose, Penguin), I had no difficulty in identifying my plant as a member of the (amusingly named) spurge family (supposedly a word derived from 'to purge' - a reference to the plant's laxitive effect). There are over a dozen British spurges however, so identifying the species involved slightly more work, but on the basis that my plant has a smooth (non-hairy) stem; leaves edged with tiny teeth, arranged in alternating fashion along the stem; and a "flower head" (umbel) with five-fold symmetry - together with a few other features relevent to the key in the book above - I'm identifying it as Sun Spurge (Euphorbia heliscopia).

Sun Spurge is normally a single-stemmed plant. Occasionally however, as in photo 2, it may banch from the base (since this plant is a common weed in my garden I felt no compunction in pulling it out to photograph it).

Photo 3 shows Sun Spurge's five-fold symmetric 'umbel'. The five leaves at the base of the umbel are known as bracts. The individual 'cups' containing the tiny central flowers are known as involucres.

An obvious feature of Sun Spurge is that the flower-head is almost entirely green. Since much of any plant's effort goes towards harvesting sunlight, it clearly makes sense to pack every available surface with green chlorphyll. What puzzles me is that most plants don't do this however. Instead they use up precious resources producing brightly-coloured flowers, the said purpose being to advertise their presence to pollinating insects (or so I understand). The question I then have however, is why Sun Spurge doesn't need to do the same? Can anyone comment?

Reminding me of piece of surrealist sculpture from an Yves Tanguy painting, Photo 4 shows a closeup (40x magnification) of one of the tiny flowers of Sun Spurge, peppered with yellow pollen grains. My understanding is that the small, green, plate-like 'petals' nearest to you are nectar producing glands. The round object in the background is the plant's ovary-containing female fruit. The three forked prongs sticking up from it are stigmas. A pollen grain landing on one of these will fertilise the ovary, causing the fruit to swell up, eventually to the point of bursting when it explosively scatters seed over the surrounding soil.

Sun spurge is part of the large Euphorbiaceae family of plants comprising some 7,50o species. The leaves are a favoured food of the caterpillars of the Spurge Hawk moth (a migrant visitor to Southern Britain). When broken, the stem bleeds a milky-white sap and from my copy of Medicinal Plants In Folk Tradition (Allen and Hatfield, Timber Press) I learn that our ancestors used the sap to cure warts. I certainly don't advise anyone try this however since the sap is a serious irritant and worse, a carcinogen. If that isn't enough to deter casual experimentation, as the book describes, one man given a dose 'as a joke' (!) in the ninteenth century:

'ran up and down the street like a madman, and swelled so big that his friends had to bind him round with hay-ropes lest he shall burst'

With friends like that who needs enemies!