698 



WEED FLORA OF IOWA 



grass {Eleusine indica), Sida rhombifolia, spiny clotbur {XantMum 

 spinosum), zinnia {Zinnia pauciflora). 



Fig. 538. Spiny Clotbur. An abundant and troublesome weed In South Africa, 



occasionally a weed in southeastern Iowa. 



(After Thurber.) 



As an example of the spread of American weeds in Europe sev- 

 eral weeds may be cited. The small ragweed {Ambrosia artemisiae- 

 f olia) , -which is common throughout the United States, has accord- 

 ing to Laubert, been widely spread with American grown clover 

 seed*, though known in isolated places in Germany for forty years. 

 It is now on the increase, perhaps because of the recent clover 

 importation, yet its spread is somewhat limited because it blooms 

 late. The plants are often only pistillate. In Steglitz, Germany, 

 it was associated with black bindweed {Polygonum convolvulus), 

 fleabane {Erigeron canadense), wall barley {Hordeum murinum), 

 ribgrass {Plantago lanceolata) , common plantain {Plantago major), 

 wild carrot {Daucus carota), yarrow {Achillea millefolium) and 

 tumble weed {Corispermum hyssopifolium) . 



•Landw. Jahrbucher. 35: 735-737. 



