Tuesday, June 23, 2009

Killer Teasels?


Over a century ago there was considerable debate as to whether the teasel Dipsacus fullonum was a carnivorous plant. It’s not difficult to see why – the bases of the leaves clasp the stem to form a large cup that fills with rainwater and this soon fills with drowned insects. The criteria for a plant being truly carnivorous are that it must lure its prey, trap it, kill it, digest the victim and absorb the nutrients that it releases. Teasel fails on the fourth of these criteria and probably the fifth too – it doesn’t secrete any enzymes of its own to digest the prey and there’s no clear evidence that it absorbs any nutrients from the soup of rotting flies in its drowning pools. But it can certainly lure and trap its prey, in much the same way as a pitcher plant. This bumblebee was presumably attracted by the water in the leaf bases and as soon as it landed it slipped into the water. From that moment on it was doomed. It struggled to climb out but its wet feet slipped on the shiny, smooth surface of the stem and leaf bases. I thought about rescuing it but within less than half a minute it had fallen back in, become totally saturated and had drowned. The teasel leaf bases soon fill with dead insects and must also have a rich population of bacteria and fungi that can digest them, so maybe it’s time for another look at the eating habits of this plant, to check again at whether it derives any nutritional benefits from its decaying captures. Birds certainly do – I’ve sometimes seem blue tits and great tits raiding this gruesome larder.

6 comments:

  1. The very same Teasel loved by Goldfinches?
    I shall see this plant in a new light now.
    Fascinating Phil.

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  2. Amazing photos,and a new concept for me. Thanks for sharing-

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  3. Hi Keith, You're right about the goldfinches loving them. And in spite of the fact that they down a few insects, those globular empty seed heads do shelter a host of small insects in winter.

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  4. Thanks for dropping by Cheryl, do you have teasels in Ohio?

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  5. Hi Phil, I am not sure if you have come across a recent, free access paper showing that supplementation with insects increases seed set - but not plant growth - in Dipsacus:
    http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0017935

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  6. Thanks Blackbird - that's fascinating. I remembered reading an old account of its purported carnivorous tendencies when I was browsing old journals in our library, about 25 years ago - looking at the references in this paper I realise it was the Christy 1923 paper. I wonder if one could use fluorescent tracers like fluorescein to trace the pathway of uptake? Thanks again - greatly appreciated...

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