Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles can also occur all over the plant – on the stem and on the flat parts of the leaves. These prickles protect the plant from herbivores. Typically, an involucre with a clasping shape similar to a cup or urn subtends each of a thistle's flower heads. The typically feathery pappus of a ripe thistle flower is known as thistle-down.
Milk thistle flowerhead
Cirsium arizonicum, showing arachnoid cobwebbiness on stems and leaves, with ants attending aphids that might be taking advantage of the shelter.
Thistles on Calton Hill in central Edinburgh, Scotland, 2023
Carduus nutans in the early morning light.
The family Asteraceae, with the original name Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae were first described in the year 1740. The number of species in Asteraceae is rivaled only by the Orchidaceae, and which is the larger family is unclear as the quantity of extant species in each family is unknown.
Asteraceae
A typical Asteraceae flower head showing the (five) individual ray florets and the (approximately 16) disk florets of a specimen of (Bidens torta)
Discoid flowerheads of Delairea odorata.
Anemochory in Carlina