COMPOSITES 437 



or alternate, thin, once- or twice-pinnatifid: fruit or bur globular, with 

 spines. Roadsides and waste places. 



A. trifida, Linn. Great ragweed. Three to 12 ft., with opposite 3-lobed 

 serrate leaves: fruit or bur obovate, with 5 or 6 tubercles. Swales. 



9. AGERATUM. Ageratum. 



Small diffuse mostly hairy herbs, with opposite simple leaves: heads 

 small, blue, white or rose, rayless, the involucre cup-shaped and composed 

 of narrow bracts; torus flattish; pappus of a few rough bristles. 



A. conyzoides, Linn. (A. mexichnum of gardens). Annual pubescent 

 herb, with ovate-deltoid serrate leaves: cultivated (from tropical America) 

 for small and numerous clustered soft heads. 



10. DAHLIA. 



Stout familiar garden herbs, tall and branching, from tuberous roots: 

 leaves opposite, pinnately divided: ray flowers in natural state are neutral 

 or pistillate and fertile; disk flowers perfect; involucre double, outer scales 

 distinct and leaf-like, the inner united at base; receptacle chaffy; pappus 

 none. In the big cultivated dahlias, most of the flowers are rays. 



D. variabilis, Desf. Figs. 257, 258. Several feet, with fine large heads 

 of flowers, colors various; heads solitary: leaves pinnate, leaflets unequal, 

 3-7, ovate-acuminate, coarsely serrate. Mexico. 



11. C6SMOS. 



Handsome tall plants, 4-5 ft. high, cultivated for the fine foliage and late 

 flowers: leaves opposite, very finely dissected, thrice-compound, the leaflets 

 extremely narrow, and sessile: flower-head with double involucre; the outer 

 bracts dark green, calyx-like, 8 in number, the inner scales erect, with 

 recurved tips; ray flowers, usually 8, neutral, white, pink; disk flowers per- 

 fect, tubular, yellow; receptacle chaffy: achenes flattened, beaked. Mexico. 



C. bipinnatus, Cav. Rays 1-2 in. long, crimson, rose or white, the disk 

 yellow. The commonest species. 



C. sulphureus, Cav. Both rays and disk yellow. 



12. ACHILLEA. Yarrow. 



Low perennial or annual herbs: heads corymbose, many-flowered, white 

 or rose, with fertile rays; scales of involucre overlapping (imbricated) ; torus 

 flattish, chaffy; pappus none. 



A. Millefolium, Linn. Yarrow. Stems simple below, but branching at 

 the top into a large rather dense umbel-like flower-cluster: leaves very 

 dark green, twice pinnatifid into very fine divisions: rays 4-5. Fields 

 everywhere. 



13. ANTHEMIS. Chamomile. Fig. 417. 



Strong-scented, branching herbs with finely pinnatifid leaves and 

 many-flowered heads, solitary on peduncles: ray flowers white or yellow, 

 pistillate or neutral, the edge of corolla entire or "2-3-toothed: disk flowers 



