SCATTERING OF WEEDS 



633 



goosefoot (Chenopodiaceae) and the buckwheat (Polygonaceae) 

 are recognized as cohorts of marauders thronging the highways, 

 forging into fields, contesting with crops and contributing little 

 beauty and few representatives of economic value. The great cos- 

 mopolitan sunflower family {Gompositae) contributes numerous 

 species of economic reputation as well as a goodly number of widely 

 recognized but combated species. 



A large number of plants may be identified with their families 

 by their type of fruit structure only. Families which have little 

 variation in fruit structure have few agents of dissemination, while 

 families with considerable variation have usually several agents, 

 as in the sunflower family in which the calyx is represented by 

 bristles forming a parachute in the dandelion, lettuce or the 



Fig. 496A Fig. 496B 



Fig. 496. Fruit scattered by the wind and animals. A. To the right 

 scattered by the wind: a, achene of Goldenrod (SoUdago rigida) ; b, Blue 

 Flowered Lettuce (Lacfuca floridana), the bristly hairs called the pappus. 

 To the left scattered by animals : a, an achene, commonly called a seed, of 

 _^ Bootjack or Beggar-ticks (.Bidens frondosa) ; b, Spanish Needle (Bidems 

 hipinnata) . 



B. Anemone iAnemone cylindrica) . 

 (Drawings by Charlotte M. King, la. Agr. Exp. Sta.) 



Canada thistle, or terminating in teeth as in Bidens or becoming 

 a spiny involucre as in the cocklebur and the burdock. Some 

 representatives of this family, for example the ragweed, have no 

 special means of transportation and the seeds fall in great quan- 

 tities near the mother plant where they germinate in large num- 

 bers if not distributed by wind-driven snow or by chance inclusion 

 with agricultural seeds. . The seeds of the milkweed family 



