Peas in a pod

, 26 July 2013

common bird’s-foot-trefoil common bird’s-foot-trefoil

Author Kevin Lerwill

Gatwick Greenspace Community Project Officer

If you wander out onto a heathland on a hot summer’s day and stand quietly next to a clump of gorse (Ulex europaeus) you will hear the ‘cracking’ sound of the ripe seed pods exploding (or dehiscing) in the heat and dispersing their contents. It was this experience that I was enjoying the other day, and also a flush of yellows and purples from the trefoils and vetches that caught the eye while I was leading a walk through meadows managed for wild flowers, that got me wanting to know a little more about the pea family.

The Fabaceae or Leguminosae as they are known, are well represented in the UK with around 80 species. They come in a great range of shapes and sizes from the shrubby gorses, whins and brooms, to the herbaceous climbing and scrambling ones such as tufted vetch (Vicia cracca) and meadow vetchling (Lathyrus pratensis), and to the low-growing, prostrate and creeping such as common bird’s-foot-trefoil (Lotus corniculatus) white clover (Trifolium repens), and restharrow (Ononis repens). Some are common and some are rare, but either way their brightly coloured flowers are a welcome sight and scent in the landscape, providing excellent nectar sources to a whole host of insects, with many insects also depending on the plants to complete their life cycles – the larva of six-spot burnet moths feed on common birds-foot-trefoil, while the adonis blue butterfly chooses horseshoe vetch (Hippocrepis comosa) on which to lay its eggs, to pick out just two examples.

But what do the pea species have in common with each other? Many are ‘nitrogen fixers’, meaning that they host bacteria known as rhizobia in their roots which are able to take nitrogen from the air and convert it into nitrates which are usable by the host plant – a great example of symbiosis. This trick is not lost on farmers and gardeners who sow pea crops to help replenish the soil. You can often recognise a pea out of flower by its pinnate or trifoliate leaves, but is the shape of the flowers that is so familiar. The five petals are typically arranged with an erect standard or banner at the top, two wind petals at the sides and the two lower petals forming a boat-shaped keel. This shape makes them irresistible to bees with explosive consequences in some cases. When the bee alights on the lip of the gorse flower, the join between the keel petals is broken, releasing the stamens, which are coiled spring-like for this very moment, to shoot pollen onto the bee. The fruits are typically an elongated pod, which splits into two valves to release the seeds, and when you look at how the pods of common bird’s-foot-trefoil are arranged it is easy to see how the flower got its name.

So get out there and take a closer look, listen and sniff, before the flowering season is over.

They are all they are cracked up to pea.

gorse / Richard Cobden gorse / Richard Cobden

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  • Mark Hedgecock:

    Glad it’s not just me that likes the pea family! I hope this “pea“ques (!) some interest in others.

    07 Aug 2013 21:52:29