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Wednesday, March 31, 2010

Hellebores Helleborus spp.

Hellebores are popular garden plants because they flower so early in the year. The so-called Lenten rose (Helleborus orientalis and its many hybrids that are grouped together as H. x hybridus) - above -  flowers in March in British gardens and H.niger a.k.a. the Christmas rose (below) flowers even earlier, though never earlier than February in my Durham garden. Both are also popular with the first bumblebees to emerge from hibernation, thanks to a peculiarity of their floral architecture.

Hellebore flowers are deceptive because those petals are not petals at all - they're the sepals that protected the bud before the flower opened, that then expand and assume a petal-like advertising function when the plant blooms. The real petals are converted into a ring of green, tube-like structures with nectar-secreting tissue inside.

Once the flower opens these tubular petals fill with nectar and become a favoured refuelling station for bumblebees. The stamens, as you can see in the photo above, burst open a few at a time, and the nectaries keep filling with nectar until all the pollen has been released and the stigmas - in the centre of the flower - have been pollinated. True petals usually fall from a flower once it has been fertilised and seeds begin to set, but the sepals of hellebores remain and become green, photosynthesising and providing resources for the developing seeds...... as in the bottom left illustration below.
Illustration source: Prof. Dr. Otto Wilhelm Thomé ''Flora von Deutschland, Österreich und der Schweiz'' 1885.
Once the seeds ripen and are shed, a further unusual feature becomes apparent...

..... because each seed has a small white body called an elaiosome attached to the seed coat that's attractive to ants, which carry the seeds away from the parent plant. The seeds have a natural dormancy that takes a cycle of winter chilling to break, before they will germinate. You can find an excellent web site devoted to this plant genus at

Sunday, March 28, 2010

Rhubarb Rheum rhabarbarum

Legend has it that the secret of 'forcing' rhubarb - keeping it in the dark to produce rapidly elongating succulent leaf stalks and to suppress green chlorophyll production - was discovered by accident at the Chelsea Physic Garden in 1815. That, combined with sugar becoming an increasingly affordable sweetener to counteract the tartness of the plant, contributed to the popularity of rhubarb and custard as a Victtorain pudding. Before that the plant had been grown as a medicinal plant on account of its somewhat devastating laxative properties, with documented use dating back to a Chinese herbal in 2700 BC. In was so effective as a laxative that in 1921 one doctor recommended it as a pugative that would even cure amoebic dysentry. Rhubarb leaves, with their high concentration of oxalic acid and other toxins, are poisonous and there have been a few documented cases of deaths after eating them, although you'd need to be rather unlucky and consume a large amount to succumb.
Today forced rhubarb is mainly grown commercially in darkened shed in the 'rhubarb triangle' around Wakefield in West Yorkshire, where it is harvested by candlelight. Plants need a winter chill to break the dormancy of the buds of the rhubarb crown. The exact parentage of domesticated rhubarb is uncertain and it's known to be a hybrid of more than one species. All told, around 50 species of Rheum are known  and at least one of the parents of the domesticated crop came from south-eastern Russia, where it grows on the banks of the River Volga.  

Friday, March 26, 2010

Common daisy - a postscript: the Childling Daisie

This delightful variation on the common daisy, known as the Childling Daisy or Hen-and-Chicks Daisy, photographed in Durham University Botanic Garden, was first described by the Elizabethan herbalist John Gerard. In his famous herbal of 1597 he wrote "There is another pretty Daisie which differs from the first described onely in the floure, which at the sides thereof puts forth many footstalks, carrying also little double flours............ so that each stalke carrieth as it were an old one and the brood thereof; whence they have fitly termed it the childling Daisie.”

Many members of the daisy family (Compositae, also known as the Asteraceae) sometimes produce additional flower heads (capitula) from the main one, when individual florets develop abnormally but this daisy - known as Bellis perennis prolifera - is significantly different, with multiple minor flowers on branches that arise from the main flower stalk, below the central flower.
Today plant molecular biologists working at the cutting edge of science depend on being able to generate mutants of plants at will in order to study and understand the way that genes work. Our Elizabethan ancestors were equally fascinated by these unusual ‘sports’ of wild flowers and cultivated them in their gardens ....... where some are still grown as horticultural curiosities.

Saturday, March 20, 2010

Common Daisy Bellis perennis

I’m not much of a fan of billiard table-perfect lawns. Striving to maintain monotonous grass in a perpetual state of arrested development seems to me to be the height of horticultural futility, especially when you consider the vast quantities of fertiliser and lawn mower fuel used and the suburban noise pollution that comes from lawnmowers. There are many more interesting and creative ways to cultivate a patch of land. Dr. Lalita Calabria has some interesting ideas over on her Adventures of a Phytochemist blog. But, if it wasn’t for mown grass, daisies would be a lot less common than they are – and I rather like daisies. They’re well-adapted to life in lawns, with rosettes of leaves pressed so close to the soil that mower blades can’t do them much damage and even it they do lose their flower heads new ones spring up very quickly, pretty much all-year-round. Daisies thrive in the open, low-diversity lawn habitat but soon go into decline if you stop mowing the grass – they can’t cope with deep shade of tall grasses.
In about a month’s time roadside strips of grass around towns and villages will be enlivened with cheerful displays of daisies – and they’ll stay like that for a week or two until the contractors arrive for the annual spring mowing. The daisies will be decapitated but close-cropping the grass like this will ensure the daisy plants' survival for the rest of the year: one of life’s little ironies.

Monday, March 15, 2010

Jade Vine Strongylodon macrobotrys

I first encountered this sensational plant at the Chelsea Flower Show in 1973, when I was a student, and didn't see it again until I visited its native country - the Philippines - in the late 1980s. I don't think I've seen another plant with a flower colour that can match it. It's now an endangered species, disappearing along with its tropical rainforest habitat. The plant is a climber and the flowers are pollinated by bats. The large racemes of flowers are pollinated by bats and dangle from long racemes from below the tree canopy - a habit known botanically as flagelliflory, that provides an uninterrupted flight path for bat pollinators.
 Fortunately we now have a plant flourishing in the Botanic Garden tropical house at Durham University. This is the first raceme of flowers to come into bloom this year and there are about another twenty or so still in bud, so it will be putting on a fine display over the coming weeks. 

Jade vine's flowers are typical of the pea family, with a standard petal and two wing petals on either side of a keel petal that's shaped like a claw. Inside the keel lie the stamens and the stigma at the tip of a long style. When a bat probes for nectar in the throat of the flower it pushes downwards on the keel petal and the stigma protrudes, forcing out a plug of pollen onto the bat's fur. It's a pity we don't have any suitable resident bats in our glashouse.

There's also a fine specimen growing in the glasshouses at Roundhay Park near Leeds and another at Cambridge University Botanic Garden.

Saturday, March 13, 2010

Hymenocallis x festalis Amaryllidaceae

There seems to be quite a lot of confusion about the name of this fragrant lily. You'll sometimes seem it listed as Ismene x festalis, spider lily, Peruvian daffodil and also Peruvian lily. There are over 60 species of Hymenocallis, that come from South America and the southern United States. This example is a hybrid (denoted by the x between the generic and specific names) between H. longipetala and H. narcissiflora and is notable for its long stamens and very long style. When a pollen grain lands on the stigma and germinates the pollen tube that it produces has to grow over 10cm. through that style to reach and fertilise the ovules - a remarkable length for what is effectively a single cell. I've only grown this bulbous plant once but it thrived in a cool greenhouse.

Andrea has posted a lovely photograph of another species, Hymenocallis littoralis, on her blog.

Tuesday, March 9, 2010

Sarracenia spp. Trumpet Pitcher

It's understandable that people admire Sarracenia plants for their funnel-shaped pitfall traps that catch and drown insects but in many ways their flowers are equally remarkable..... and their purpose is to lure insects for pollination rather than as a food source, so this is a Jekyll-and-Hyde plant. Each flower consists of five sepals that expand as the bud opens, arching downwards above the rest of the floral structure.

Below hang five petals, shaped like tongues, protruding from gaps in the weird, inverted umbrella-shaped stigma that is suspended from a style in the centre of the flower that leads to the ovary and ovules (hidden from view in these photographs). There's a very fine cutaway illustration here that identifies all the key structures. Bees crawl along the petal and in through those curved indentations that you can see around the edge of the 'inverted umbrella'. Once inside they collect pollen or deliver it to the tips of one of the five receptive stigmas that are at the apex of each of the upwards curving, greenish bands of thickening visible between the petals. It can be quite a struggle for the bees to force their way in and out again.

When they are in full flower the blooms have a scent that's reminiscent of a bed of stinging nettles on a hot summer's day, although in the green-flowered species above (Sarracenia flava) it smells of cat urine - which must be attractive to bees but makes this plant unsuitable for growing on your kitchen windowsill.

The flowers appear in spring, before a new crop of pitchers is produced, and in the bud stage they're reminiscent of a rather elegant art deco lamp. The flowers are a nectar source for bees but the pitchers are also a potential death trap, so the blooms are elevated in long stems well above the deadly pitchers. The scent of the flowers seems to be a powerful attractant to bees - if I leave my conservatory doors open during the flowering period, in early spring, the room soon fills with eager bumblebees, newly emerged from hibernation and desperate to get at the flowers.

Sarracenia species are native to North America. Some species are hardy (although the winter we've just had will have put that statement to the test) and S.purpurea is naturalised in the wild in Ireland and in the Lake District.

For more information about carnivorous plants take a look at

Thursday, March 4, 2010

Sesame, Sesamum indicum

Sesame Sesame indicum is reminiscent of a small white foxglove and is an easy plant to grow from the culinary sesame seeds that you can buy in the supermarket. Some say that it is the most ancient of all the oil crops and evidence from the Middle East suggests that it has been cultivated there for over 4000 years, prized for its nutty oil whose flavour becomes more intense when the seeds are roasted - which is why it's used to garnish loaves before baking. The oil can constitute up to 60 per cent of the weight of the seed. The crop probably spread from Africa to the Middle East and then throughout Asia. Today India is a major producer but you can raise your own crop in a greenhouse, conservatory or on a sunny windowsill by sowing a few seeds. They'll begin to ripen seed pods in about 10-12 weeks from sowing.