This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Annual rather coarse herbs, with pubescent foliage and erect or creeping stems, opposite, broad, often leathery, toothed leaves, and radiate but inconspicuous heads, axillary to leaflike bracts. Involucre double, an outer one of flat herbaceous bracts, and an inner one of several smaller bracts which become bur-like and fall away enclosing an achene at maturity. Receptacle concave or convex. Ray-flowers few, in 1 series, the rays very small, yellowish, concave or hooded. Disk-flowers perfect, sterile. Anthers entire at the base. Achenes broadest above the middle, slightly curved. Pappus wanting. [Greek, thorn-seed, from the prickly, bur-like fruits.]
About 3 species, natives of tropical America. Type species: Acanthospermum brasilium Schrank.
Melampodium australe Loefl. Iter. Hisp. 268. 1758.
A. xanthoides DC. Prodr. 5: 521. 1836.
A. australe Kuntze. Rev. Gen. Pl. 303. 1891.
Stems branching at the base, the branches prostrate or creeping, 1/3°-2° long. Leaves ovate, oval or rhombic, 1/3'-1' long, usually acute, serrate-dentate above the middle, cu-neate at the base; peduncles shorter than the subtending leaves; involucre campanulate, the bracts broadly ovate, obtuse, 1"-1 1/2" long, uniformly prickly; disk-flowers with corollas 1" long, puberulent; mature inner bracts of the involucre forming a starfish-like bur, each lobe 4"-5" long, densely beset with uniform blunt weak prickles.
In waste places and dry soil, Virginia to Florida and Louisiana. June-Oct. Widely distributed as a weed in tropical regions.