FAMILY: Compositae (Asteraceae)—the Daisy Family
The largest family of vascular plants, the Compositae are distributed worldwide and are economically very important. Food members of the family include lettuce (Lactuca), endive, escarole, and chicory (Cichorium); artichoke (Cynara), salsify (Tragopogon), and sunfowers (Helianthus). Numerous species are used as ornamentals; especially notable are Aster, Chrysanthemum, daisies, cosmos, dahlia, strawflowers, cineraria, marigolds, zinnia, globe thistle, and edelweiss. Composite weeds often have detrimental impact. A nonexhaustive list would include ragweed, various thistles, horse weed (Conyza), galinsoga, fleabane, goldenrod, beggarticks, sowthistle, dandelion, and a host of less numerous taxa. A small proportion of the Compositae are poisonous and are detailed as separate entries (see Arctium spp., Eupatorium rugosum, Helenium autumnale, Tanacetum vulgare, and Xanthium spp.). Senecio is mentioned at the end of this entry.
Flowers of the Compositae are aggregated in close heads, on a receptacle, and surrounded by involucral bracts that are usually green. The ovary is inferior. The calyx is modified into a pappus, which crowns the summit of the ovary in the form of bristles, awns, scales, or teeth, or is absent. The corolla is either ligulate (flat and strap-shaped ray-flowers), or tubular (disk-flowers, often opening to form a 5-pointed star). The heads can be composed of all ray-flowers (heads ligulate) or all disk-flowers (heads discoid), or with ray-flowers along the margin and disk-flowers in the center (heads radiate). The stamens generally number 5, are fused upon the corolla, and bear anthers that are united into a tube. The style is 2- cleft; the ovary matures into a fruit, the achene, which contains a single, erect seed.
PHENOLOGY: Arctium species flower July through October.
DISTRIBUTION: The four Arctium species found in the Commonwealth occur in waste places, disturbed habitats, and roadsides.
PLANT CHARACTERISTICS: In the genus Arctium the heads are entirely discoid; involucres: globular; involucral bracts: attenuate to long, stiff, hooked tips; pappus: numerous, rough, separate, deciduous bristles; leaves: large and coarse.
POISONOUS PARTS: The green, above-ground portions may cause contact dermatitis.
SYMPTOMS: Contact may cause itching, burning, or reddening of the skin.
POISONOUS PRINCIPLES: The agents that cause rash upon contact are unknown but probably are lactones (perhaps sesquiterpenes).
CONFUSED TAXA: Our four species of Arctium are: great burdock (A. Lappa L.). which has strongly angled leaf petioles with solid centers; hairy burdock (A. tomentosum Mill.), which has hollow leaf petioles and small flower heads (2.0-2.5 cm broad); the woodland burdock (A. nemorosum Lej & Court.) has hollow leaf petioles and larger flower heads (2 5-3.5 cm broad); and the common burdock (A minus (Hill) Bernh.), which has the smallest flower heads (1.2-2.5 cm broad). Several varieties of the common burdock, have been described by researchers based on leaf shape and corolla color. The burdocks are sometimes confused with the cockleburs (see Xanthium) and rhubarb (see Rheum).
SPECIES OF ANIMALS AFFECTED: Possibly only humans are susceptible to skin reactions from contact with the lactones in burdock.
TREATMENT: (23); (26)
OF INTEREST: Burdocks have been used in folk remedies for various ailments. Arctium may have hypoglycemic activity and therefore have potential as a medicine for diabetes. The Meskwaki Indians used A. Lappa root as an aid in childbirth, and 17th Century Europeans used it as a putative remedy for venereal disease.
Several species of Senecio (groundsel, ragwort), also in the family Compositae, produce pyrrolizidine alkaloids similar to those produced by Crotalaria (rattlebox), in the bean family. Of the Senecio species suspected or known to be toxic, only S. vulgaris L. occurs in Pennsylvania and its toxicity has not been proven in North America. The toxic groundsels are a problem mainly on western rangelands. Senecio toxicosis is similar to that described for Crotalaria.