Sunday, 17 July 2011

Peloria

Linaria vulgaris f. peloria
 In 1742 a Swedish student, M. Liöberg, found a strange plant growing near Stockholm. In its vegetative characters it looked just like the common Yellow Toadflax, which his professor named, in his new binomial system, Linaria vulgaris. But this plant was very different. Instead of the familiar bilaterally symmetric toadflax flowers, those of this plant were really bizarre-looking, with five spurs instead of one, a long tube and a rounded, five-lobed object at the top, quite unlike the usual bunny-shaped flowers of a toadflax. He took material to Linnaeus for the great man's opinion and it caused him quite a stir of consternation. Linnaeus was a creationist and assumed that species were immutable, but this plant was a problem.

Normal Linaria vulgaris
In his own 'sexual system' for classifying plants by their reproductive parts Linnaeus had placed Linaria vulgaris in the section Didynamia, as it had two unequal pairs of stamens. This odd plant, however, had radially symmetrical flowers, with five anthers matching the five spurs. This placed it into the class Pentandria, along with other plants from many families having five stamens. But its evident connection with Linaria gave Linnaeus pause. How could such a difference have come about? There was no genetic framework to think in and mutations that were known could be easily understood - differently coloured flowers and so on - but such a change in a flower's shape was unheard of. In the end Linnaeus decided it must be a hybrid between Linaria and some other (unknown) plant, and named it Peloria, from the Greek for a monster, but in so doing he had to confront the philosphical problem that this meant that not all plants had been placed on Earth by the Creator.

A peloric Phalaenopsis
(img: M. Bishop)
Now understood as a mutant under the name Linaria vulgaris forma peloria, this odd plant has been used by evolutionary biologists from Darwin onwards as an example of how mutation can lead to change in morphology; peloric flowers occur in many genera and can cause them also to look quite bizarre. A radially symmetrical terminal flower is common in Digitalis, for example, and it occurs in orchids too, often with dramatic effect. The phenomenon is of great interest to those studying developmental pathways, and it's clearly not only, or not always, a simple mutation that causes it.

Peloric Linaria purpurea
In October 2010, on the last occasion that I was able to visit the garden of the late Elizabeth Parker-Jervis, I spotted a peloric flower or two on a plant of white-flowered Linaria purpurea, so dug it up and brought it home. Cuttings rooted well over winter, so I now have several plants, all flowering. What has surprised me is that while all inflorescences show some pelorism, not all flowers show it, with many being entirely normal and some being intermediate, with two or three spurs on an otherwise more normally shaped flower. There is certainly no simple mutation here, but a change in genetic expression as the inflorescence develops. This needs laboratory investigation, but on a manageable, horticultural scale I am keen to see what happens in the next generation, or generations. The normal flowers mean that normal pollination can occur, so seed will be set, whereas in peloric flowers in Linaria the curious floral shape precludes the normal pollination process and no seed develops, if artificial pollination is not performed. Now I'm thinking I should get busy with my paintbrush...

A peloric form of Linaria purpurea has been recorded before, but it is an unusual phenomenon. I wonder how often it has occurred in L. vulgaris: I notice some differences between online images, so there may be different clones, but I suspect that this too is a rare occurrence. L. vulgaris is a vigorously spreading perennial maintaining itself by vegetative growth, meaning that it is easily propagated and maintained, whereas L. purpurea is a short-lived perennial at best. Not many people would welcome normal L. vulgaris into the garden because of this rampancy, but forma peloria is desirable for its curiosity and, it has to be said, it contributes a very useful patch of colour. Only one nursery currently offer it it in the UK: this is a plant that should be more widely grown.

Linaria vulgaris f. peloria at Colesbourne Park

7 comments:

  1. Fascinating stuff! I'd never come across this phenomenon before - very interesting.

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  2. Do you think this plant from Elizabeth Parker-Jervis traces back to Bowles's Lunatic Garden? Would seem probable.
    Ditto the previous comment.

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  3. What a fascinating plant/story! I love toadflax! I know it's a thug - but if you think your plant should be more widely grown - we out here will be more than pleased to grow it!

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  4. Thank you for the name of this strange phenomenon - I could see it on some stems of my Phalenopsis, with other stems with normal flowers.
    Marek W.

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  5. Great plant and fascinating story. Now, of course, I want one.

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  6. This was really interesting, especially because I was researching Linaria for my own blog on Mediterranean plants ( http://anentangledbank.wordpress.com/). I'm new here - I found you via Google - but I've been reading other entries and following links and enjoying it. I would like to know if you have any objection to my using your picture of the peloric Linaria on my blog? Thanks - and congratulations on your new job, too.

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  7. Dear John-

    I am writing on behalf of Sonia Sultan, Chair of Biology at Wesleyan University. Sonia is working on a book for Oxford University Press' Ecology and Evolution series, and she would love to use your linaria images as a lucid illustration of an epimutation. I tried to hunt down your e-mail but couldn't find it so I decided to contact you here.

    If you are be interesting in corresponding about this, my e-mail is frakerw@gmail.com

    I really hope to hear from you soon!

    Thank you,
    Will Fraker

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