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any of several related resinous herbs (chiefly species of Hemizonia and Madia) of the family Asteraceae (asteraster
[Gr.,=star], common name for the Asteraceae (Compositae), the aster family, in North America, name for plants of the genus Aster, sometimes called wild asters, and for a related plant more correctly called China aster (Callistephus chinensis
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 family), having strongly scented and sticky herbage. Most North American species are found in fields and on dry hillsides of the Southwest and the Pacific region. They bear daisylike heads of yellow or cream-colored flowers. The heads of the common tarweed (M. elegans, also called common madia) are marked with an inner red ring and, like those of other Madia species, open in the evening and close before noon. Several species of this genus, notably M. sativa of Chile, are cultivated as oilseeds. Similar related Western plants are the rosinweeds (Calycadenia) and the gumweeds, or sticky-heads (Grindelia). Several gumweed species have become established in the East, where they are sometimes called tarweeds. The dried herbage of some gumweeds, containing resinous substances and essential oils, has been used in domestic remedies for treating burns and ivy poisoning. Tarweeds are classified in the division MagnoliophytaMagnoliophyta
, division of the plant kingdom consisting of those organisms commonly called the flowering plants, or angiosperms. The angiosperms have leaves, stems, and roots, and vascular, or conducting, tissue (xylem and phloem).
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, class Magnoliopsida, order Asterales, family Asteraceae.
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Initial results indicate that while stinkwort does eventually grow roots as deep as yellow starthistle and virgate tarweed, this occurs several weeks after these other grassland annuals grow their roots.
The resin emits a strong aromatic odor that resembles the smell of tarweeds. The flowerheads are 0.2 to 0.3 inch (5 to 7 millimeters) in diameter and consist of short yellow ray flowers on the outer edge and yellow to reddish disk flowers in the center.
Aside from the tarweeds, there are few other late-season winter annual species with a similar life cycle in the native California flora.
The strong new evidence that a tarweed made its way from California to Hawaii also raises questions about the origins of plants outside Hawaii, he adds.
One of the silversword's tarweed ancestors is also considered an endangered species by the California Native Plant Protection Society.
The California tarweeds, 99 species of scrubby plants that Kyhos says most people "probably would walk right by" without noticing, resemble Hawaii's silverswords in several important ways.
But Kyhos and Carr's attempts to cross numerous different tarweeds to silversword plants proved fruitless.
Carr (eds.), Tarweeds and silverswords: evolution in the Madiinae (Asteraceae).