Word of the Week: Pappus

pappus [ papuhs ] noun, plural pappi [ papahy ]: an appendage of the cypsela (fruit) of the Asteraceae consisting of one to many bristles, awns, scales, setae, or coronas that assists in seed dispersal

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The word pappus might bring to mind dandelions and the “puff ball” seed heads you may have played with in your youth. However, if the word conjures up the silky “parachutes” of milkweed, think again. Although both appendages aid in the dispersal of their seeds, a pappus is an appendage to cypsela, the fruit of plants in the Asteraceae (aster family). Dandelion is a member of the aster family; milkweed is not.*

Some plants can regulate the dispersal of their seeds in response to the environment, modifying their structures to protect seeds until conditions favor optimal dissemination. Water often inhibits release and spurs structural changes. For example, a pine seed cone can close its scales to keep its seeds viable during wet weather and open them when the weather is warm and dry enough for seed dispersal. Likewise, many pappus-bearing plants respond to moisture. The dandelion can close the filaments of its pappi when the relative humidity levels are greater than 70% and then open them when drier, more favorable conditions exist. “Closing the pappus in response to moisture may maintain dispersal distances in dry conditions by biasing seed release in favour of greater wind speeds, while retaining some seeds in wet conditions to aid germination.” ( Seale et al., 2019) Not all cypsela with a pappus scatter on the wind, anemochory. Some, like those with awns, travel by way of epizoochory (seed dispersal on the outside of animals, usually mammals) by fortuitously attaching themselves to (usually) the hair or fur of animals, or the clothing of people.

Although pappus structures can vary greatly from tribe to tribe, they appear the same among species within a genus. Some members of the aster family, like Achillea (yarrow) and some species of Coreopsis (tickseed), Rudbeckia (yellow coneflower) and Silphium (cup plant), do not have pappi.

There are five primary types of pappus structures each with two or more subtypes:

  • bristles, which can be arranged in one or more rows and range from feathery (subplumose or plumose) to barbed (capillary barbellate or scabrous barbellate)
  • awns, with marginal barbs directed upward or backwards
  • scales, which can be paleaceous; non paleaceous; “cartilaginous stiff twin hair like scales; and multicellular hair like scales.” (Mukherjee, 2001)
  • setae, which can be plumose or non-plumose
  • coronas, which can be cupular (cup shaped) or non-cupular

* The Flora of Virginia describes the pappi of Taraxacum (dandelion) as “50–100+ barbellulate bristles.” Its Asclepias (milkweed) description includes the phrase “usually comose [bearing a tuft of hairs] at the hilum [the scar indicating where the seed was attached to the ovary].”

References

Mukherjee S, Sarkar AK. 2001. Morphological diversity of pappus in the sub family Asteroideae (Asteraceae). Journal of Economic & Taxonomic Botany. 19: 200 – 252.

Reznicek AA, Voss EG, Walters BS. 2011. Asteraceae. Michigan Flora Online. University of Michigan. [accessed 18 June 2021].

Seale M, Zhdanov O, Cummins C, Kroll E, Blatt M, Zare-Behtash H, Busse A, Mastropaolo E, Viola IM, Nakayama N. Informed dispersal of the dandelion. bioRxiv 542696. posted August 26, 2020. DOI: 10.1101/542696.

Seale M, Zhdanov O, Cummins C, Kroll E, Blatt M, Zare-Behtash H, Busse A, Mastropaolo E, Viola IM, Nakayama N. Moisture-dependent morphing tunes the dispersal of dandelion diaspores. posted February 7, 2019.

Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas. pages 254, 403.

 

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