[Illustration: Fig. 180.] =Great Ragweed.= _Ambrosia trifida_ L. Achenes hard, brown, more or less mottled, top-shaped, 7-8 mm. besides the stout beak 2-3 mm. long, sides with 5 stout ridges terminating in 5 short teeth. Native to the United States. River bottoms, low land, sometimes 15 ft. high. [Illustration: Fig. 181.] =Corn Camomile=. _Anthemis arvensis_ L. Achenes very variable, creamy white to light brown, oblong, wedge-shaped in outline, circular to four-angled in cross-section, more or less ribbed lengthwise, a ripple-shaped scar at the narrow end; apex truncate with a minute projection in the center, often with a narrow ridge about the margin. About 1.7 mm. long. Introduced from Europe. Seldom troublesome in Michigan. [Illustration: Fig. 182.] =May-weed. Dog's-Fennel.= _Anthemis Cotula_ L. Outer flowers white; achenes straw color to light brown, obovoid (large end uppermost) to oblong, circular in outline, 1.3-1.8 mm. long, with 10 warty ribs. Introduced from Europe. Old roads and waste places. [Illustration: Fig. 183.] =Great Burdock.= _Arctium Lappa._ So far as I have seen the achenes of this species, when compared with A. minus, are darker colored, rather longer, the ribs more distinct. =Smaller Burdock.= _Arctium minus_ Beruh. Possibly only a variety of _A. Lappa_ L., but the prevailing plant in central Michigan. I see no way of distinguishing the achenes of one from the other; but it makes little difference as one burdock is as bad as another. Flowers purple; achenes dull brown, often spotted with black, straight or curved, slightly flattened, oblong-prismatic with 3-5 narrowly ridged angles, and occasionally other smaller ridges, 4.5-6 mm. long. Introduced from Europe. [Illustration: Fig. 184.] =Biennial Wormwood.= _Artemisia biennis_ Willd. Achenes dark brown, smooth, somewhat flattened, 3-4 angled, obovate, narrowed at the base .8-1.3 mm. long. Native in the northwestern United States and introduced east with grass seeds. Moist land. [Illustration: Fig. 185.] =Smaller Bur-Marigold.= _Bidens cernua_ L. Flowers yellow; achenes 4-6 mm. long, dull brown, the awns lighter, flattened, 4-angled, wedge-shaped, awns 2-4, barbed downward as also are the ribs. Native of this country. Low lands. [Illustration: Fig. 186.] =Purple-stemmed Swamp Beggar-ticks.= _Bidens connata_ Muhl. Flowers orange; achenes brown, wedge-shaped or obovate, hairy, tubercled, flattened, 4-angled, 4-toothed, 4-6 mm. long, achenes and teeth downwardly barbed. Swamps, common. [Illustration: Fig. 187.] =Beggar-ticks.= _Bidens frondosa_ L. Achenes dull brown, tubercled, much flattened, obovate or oval, 6-12 mm. long, awns usually 2, spreading barbed downward. Low lands. [Illustration: Fig. 188.]