Identifying British plant families: Dr M’s top family cameos – Asteraceae

Dr M has already posted on the top 20 UK plant families and the series is on-going.

Here Dr M continues a series of cameos on a selection of these families together with his top tips for family recognition.

Asteraceae: the Daisy or Dandelion or Composite or Aster family.

A large and diverse family but the really key feature is the composite inflorescence. Yes, those big colourful Daisy, Thistle and Dandelion “flowers” are not flowers at all but an inflorescence which is an aggregation of many very small flowers (florets) all packed together in the composite inflorescence known as a capitulum.

Get your eye in for the capitulum and you will be well on the way to recognising Asteraceae.

Dissect the capitulum and you will find many tiny flowers and these can be of two kinds: strap-shaped flowers known as ray or ligule florets, and disc-shaped flowers known as disc or tube florets.

Some Asteraceae have both types of flower in one inflorescence, for example Daisies (Bellis perennis) have the capitulum with white ray florets and yellow dics florets, Achillea (Yarrow) has ray and disc florets both white and Senecio (Ragwort) has ray and disc florets both yellow.

Other Asteraceae have the capitulum with just ray florets, e.g. Dandelions (Taraxacum spp.) and similar types of yellow composites, and some Asteraceae have just disc florets, e.g. Thistles (Cirsium spp.) and Burdock (Arctium spp.).

The capitulum is enclosed by a series of overlapping bracts called involucral bracts or phyllaries.  Thistles (Cirsium spp.) have spiny phyllaries, Burdock (Arctium) has hooked phyllaries while other Asteraceae have very ornate, dissected phyllaries, e.g. the Knapweeds (Centaurea spp).

The structure and arrangement of the phyllaries can be a very important character for identifying some genera and species. Check Dr M’s post on yellow composites for some examples.

Many Asteraceae have a pappus of hairs (modification of the sepals) which aids dispersal of “seeds” (technically dry fruits called achenes) in the wind (the “Dandelion clock” is an example).

Some Asteraceae have white or coloured latex, e.g. Sowthistles (Sonchus spp.).

Such a large and diverse group so lots and lots more to learn, but that’s enough for now!

Click here to see Dr M’s top-20 British plant families Asteraceae post for more on this family.

The featured image is Arctium minus (Burdock): note the amazing hooked phyllaries which act as barbs and attach to anything hairy/woolly like horse’s tails and sheep’s fleece and human clothing!  A great way for the plant to disperse its seeds!