Derived from the Greek word pappos, meaning ‘old man’ ….for obvious reasons. It is actually a modified calyx, which is attached to the fruit of the plant (e.g. Achene in Asteraceae). Key to understanding this feature is the fact that members of the Asteraceae have an inferior ovary (they are ‘epigynous’) which means that the ovary develops below the calyx, so that the calyx part of the flower is in a good position to facilitate dispersal of the seed (being exposed to the environment).
Pappus remains a useful identification feature for members of the Asteraceae family (although not all Asteraceae have pappus and some plants in different families have pappus too). But generally speaking, you would not be a complete fool to assume you are looking at a daisy when you witness a plant with these silky and fluffy appendages.
Pappus structure varies between different species. It can be simple, feathered, bristly or white, off-white or yellowish. Some Asteraceae species are even instantly recognisable due to them not having any pappus e.g. Lapsana communis.
The ability of the pappus to facilitate dispersal of seed is down to the geometry of the pappus and the habitat it is in. Generally speaking, the more complex the pappus filaments are then the greatest drag this will have on the wind e.g. A feathery pappus is carried further by the wind than simple pappus. The height at which the pappus is featured also influences dispersal, in that the higher the pappus is up on a plant, the longer the distance of dispersal for a given wind speed. It is also worth pointing out that in dense stands of thistles, pappus seldom travels far, but the pappus of a single goats beard or dandelion in a meadow may travel some distance.