THE STRUGGLE FOR EXISTENCE 389 



18. Cow herb,* Saponaria Vaccaria. 



19. Daisy, oxeye,* Chrysanthemum Leucanihemum. 



20. Dandelion,* Taraxacum officinale. 



21. Dock, Sumex crispus. 



22. Dog fennel,* Anthemis cotula. 



23. Fox-tail grass,* Setaria glauca. 



24. Horse-nettle, Solanum carolinense. 



25. Jamestown weed or Jimson weed,* Datura Stramonium 



or D. Tatula. 



26. Johnson grass, Andropogon halepensis. 



27. Mallow,* Malva rotundifolia. 



28. Milkweed, Asclepias Cornuti. 



29. Nettle, Urtica gracilis. 



30. Pigweed,* Amarantus retroflexus. 



31. Pigweed,* Chenopodium album. 



32. Plantain,* Plantago major. 



33. Pokeberry, Phytolacca decandra. 



34. Purslane, Portulaca oleracea. 



35. Quick-grass,* Witchgrass, Agropyrum repens. 



36. Ragweed, Ambrosia artemisimfolia. 



37. Rib grass,* Plantago lanceolata. 



38. Sand bur, Cenchrus tribuloides. 



39. Shepherd's purse,* Capsella Bursa-pastoris. 



40. Smartweed, Polygonum Hydropiper. 



41. BoTxA,* Rumex Acetosella. 



42. Spanish needles, Bidens bipinnata. 



43. Sticktights, Desmodium canadense. 



44. Thistle,* Cirsium lanceolatum, C. arvense. 



45. Yarrow, Achillea Millefolium. 



457. Origin of Weeds.^ — By far the larger propoition 

 of our weeds are not native to this coilntry. Some have 

 been brought from South America and from Asia, but 

 most of the introduced kinds come from Europe. The 

 importation of various kinds of grain and of garden-seeds, 



1 See the article " Pertinacity and Predominance of Weeds," in Scientific 

 Papers of Asa Gray, selected by C. S. Sargent, Vol. 11, pp. 234-242. 



