DEVELOPMENT OF PLANTS 



491 



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

 The marsh elder {Iva), ragweed {Ambrosia), and clot-bur and 

 cockle-bur {Xanthium). 



These three alliances are apparently the. most recently evolved 

 of the angiosperms, and owing to theii- numerous variations that 

 have been so successful in meeting the present conditions upon 



Fig. 342. A common tubular flower of the Thistle family: A, inflores- 

 cence of the ironweed {Vernonia). B, sectional view of the inflorescence, 

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

 calyx or pappus. D, fruit. 



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 

 because of their protective armor of prickles, bitter, acrid juices 

 or oils and resins. Especially to be noted is the seed-like fruit 

 with its numerous devices for distribution. All these features, 

 among others, explain their extensive distribution in open coun- 

 tries. Another advantage appears in the structure of the flower 

 and the inflorescence. The highest type of a flower, as in the case 

 of a machine, is the one that is most efficient. The flower is 

 not so elaborate as the orchid or as those of some other groups, 

 but it is superior because it accomplishes its work with greater ' 

 certainty and more economically. Note the significance of 

 these facts — the ovary of the orchid consists of three carpels 



