The Dandelion

Dandelion photoWhat better way to start off reveling in the despised plants of the world than with a post on the ubiquitous dandelion, Taraxacum officinale?

We all know that the name comes from the French dent de lion, meaning “lion’s tooth,” which refers to the jagged margins of the leaves.  Growing up, I always thought that the name was “dandy lion”, which I still think seems to fit for a flower with a mane of lion-yellow.

They are very easily recognized, but there are a few similar plants that are “false dandelions” that had me fooled until I started looking more closely.  The hawkweeds and hawksbeards have very similar-looking flowers but these plants are easily distinguished from the true dandelion by their branched flowering stems — true dandelions only have a single flower at the tops of their unbranched stalks.

And I can’t remember where I found this, but the dandelion is known as the “yellow-flowered earth nail” in the Yangtze River Valley of China because of the long taproot that anchors it in the soil. I love this.

There are many interesting things about dandelions:

  1. They are edible and so you can find them in fancy supermarkets.dandelion leavesThe leaves are a little bitter and I like them raw in salads with some balsamic vinegar.  Yum!
  2. Their flowers, like all of the flowers in sunflower family (the Asteraceae), are really composites of many smaller flowers all arranged to look like one big one.  Fuji emulsion lift cross section flowerThis makes pollinating lots of flowers all at once very efficient.
  3. They close their flowers at night and open them in the day, but only if the weather is fine.  It seems that there is an interplay between temperature and light that makes this work for the dandelion — see the wonderful review article on flower movements by Doorn and Meeteren, cited below with a link to the whole paper.  According to Doorn and Meeteren, way back in 1891 it was hypothesized that flowers might close during bad weather to avoid getting their pollen wet, which seems to make sense.head and cross-section Fuji negativeThen in 1904 it was hypothesized that flower closing occurs to exclude unwanted insects or may protect against cold at night.  Why not?
  4. Dandelion flower stems have a strong gravitropic response. If the stems are detached from the plant and held sideways for a period of time, they will reorient themselves by bending upwards until they return to a near vertical position.  The response can be seen within a couple of hours but only for stems with unopened flowers.  As soon as the flowers start to open, the response diminishes.  See Clifford and Oxlade (1991), below.
  5. They have specialized “contractile” roots that can shorten the root by collapsing some cells and can thus pull the plant back into warmer soil on cold days in Spring and Fall.  Crazy.
  6. They reproduce through “apomixis“, i.e. non-sexual reproduction through seeds.  This is a very handy way to get lots of clones out over a large distance, but is not great in the long-run (evolutionarily) for genetic diversity.  Luckily, sub-populations of dandelion can reproduce sexually, so there is some mixing of genes going on.Fuji film emulsion lift of seedheadTheir genetics get studied a lot because of this trait and there is some fun work on the interaction of genes with the environment in small populations of dandelion (e.g., Mcleod et al., 2012).  One fun result that I read about: Dandelions occurring in regularly mowed lawns have short flowering stalks and leaves that are more flat against the ground than those in fields that don’t get mowed, presumably because those are the clones that survived the best under such hostile conditions.

dandelion flowerSo, the next time you grumble about dandelions in your yard, remember that those “weeds” are complex little seed machines that have evolved (even though they do a lot of cloning of themselves) to avoid your lawnmower, hunker down when it’s cold, protect their flowers from rainstorms, and are even pretty good to eat. They deserve a moment of respect before you pluck them from the ground and toss them into the yard waste.

References

Mcleod, K. A., Scascitelli, M. & Vellend, M.  2012.  Detecting small-scale genotype–environment interactions in apomictic dandelion (Taraxacum officinale) populations.  Journal of Evolutionary Biology 25:1667–1675. PubMed link to Abstract

Clifford, P.E. and Oxlade E.L. 1991. Using Dandelion Flower Stalks for Gravitropic Studies. The American Biology Teacher 53, pp. 290-293. JSTOR link to preview

Wouter G. van Doorn and Uulke van Meeteren. 2003. Flower opening and closure: a review. Journal of Experimental Botany 54: 1801-1812. Full, free article

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