Thistle. 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 are an adaptation that protects the plant from being eaten by 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. The comparative amount of spininess varies dramatically by species. For example, Cirsium heterophyllum has minimal spininess while Cirsium spinosissimum is the opposite. Typically, species adapted to dry environments have greater spininess. The term thistle is sometimes taken to mean precisely those plants in the tribe Cardueae, especially the genera Carduus, Cirsium, and Onopordum. However, plants outside this tribe are sometimes called thistles. Biennial thistles are particularly noteworthy for their high wildlife value, producing such things as copious floral resources for pollinators, nourishing seeds for birds like the goldfinch, foliage for butterfly larvae, and down for the lining of birds' nests. A thistle is the floral emblem of Scotland and Lorraine, as well as the emblem of the Encyclop�dia Britannica. Carduus nutans in the early morning light. Thistledown, a method of seed dispersal by wind. The tiny seeds are a favourite of goldfinches and some other small birds. Cirsium horridulum found in southeast Louisiana. Thistle flowers are the favourite nectar sources of the pearl-bordered fritillary, small pearl-bordered fritillary, high brown fritillary, and dark green fritillary butterflies.Thistles and thistle-seed feeders provide important sustenance for goldfinches and are strongly favored by many butterflies besides fritillaries such as the monarch, skippers, and the various types of tiger swallowtail. Hummingbirds will feed on the flowers of the biennial species, which feature large flowers, as compared with the perennial Canada thistle. Some thistles, for example Cirsium vulgare, native to Eurasia, have been widely introduced outside their native range. Control measures include Trichosirocalus weevils. A problem with this approach, at least in North America, is that the introduced weevils may affect native thistles at least as much as the desired targets. Thistles have been said to be very important nectar sources for pollinators. Some ecological organizations, such as the Xerces Society, have attempted to raise awareness of their benefits, to counteract the general agricultural and home garden labeling of thistles as unwanted weeds. The monarch butterfly, Danaus plexippus for instance, was highlighted as traditionally relying upon taller large-flowered thistle species such as Tall thistle, Cirsium altissimum, for its migration. Although such organizations focus on the benefits of native thistles, certain non-native thistles, such as Cirsium vulgare in North America, may provide similar benefits to wildlife. Some prairie and wildflower seed production companies supply bulk seed for native North American thistle species, for wildlife habitat restoration, although availability tends to be low. Thistles are particularly valued by bumblebees for their high nectar production.