458 



THE CAMPANULALES 



vated for their showy flowers: Tickseed (Coreopsis), cone flower 

 (Rudbcckia), sunflower (Hcliauthits), cornflower (Centawea), 

 cinerarias, etc. Medicinal: Wormwood (Artemisia), one species 

 of which yields absinth, milfoil (Achillea), colt's foot (Titssi- 



SS2 5 "' 



Fig. 342. A common tubular form of flower of the thistle family : A, 

 inflorescence of the ironweed (I'cnionia). B, sectional view of the in- 

 florescence, only the outer flowers in bloom. C, enlarged view of one of 

 the flowers — c, calyx or pappus. D, fruit. 



lago), tansy (Tanacctum) , chamomile (Anthcmis), arnica, bur- 

 dock (Arctium). 



The ragweed family, Ambrosiaceae, is a small group, princi- 

 pally North American in its distribution, that have become sepa- 

 rated from the Carduaceae through degeneration. The heads 

 contain a few greatly reduced anemophilous flowers that are al- 

 ways imperfect and generally lacking in calyx or corolla, or both, 

 and characterized by free anthers. The more familiar are : The 

 marsh elder (Iva), ragweed (Ambrosia) , and clot-bur and cockle 

 bur (Xautliiitm). 



These three alliances are apparently the most recently evolved 

 of the angiosperms, and owing to their numerous variations that 

 have been so successful in meeting the present conditions upon 

 the earth, they have become the dominant plants the world over. 

 Their success can be traced to a variety of causes, as the occur- 

 rence in many genera of perennial underground stems, which 

 make them to a degree independent of climate conditions and the 

 depredations of grazing animals. A great many are avoided 



