688 WEED FLORA OF IOWA 



year to year. Commerce indeed has taken an important part in 

 the migration of weeds. Wild carrot, chicory, black medick, quack 

 grass, Canada thistle, shoo-fly and dodder are becoming more and 

 more abundant in the fields of Iowa. Many weeds, moreover, first 

 make their appearance in the vicinity of cultivated fields. In 

 nearly every case wild parsnip, tansy, shoo-fly, bouncing betty and 

 butter and eggs show the influence of cultivation. 



Pig. 531A I 



Fig. 531 A IV 



Fig. 531A VI 



Fig. 531-A. Seeds scattered with commercial seed. I. Quack Grass (Agropyron 

 repens). II. Corn Flower (Centaurea cyanus) ; scattered with flower seed, 

 a frequent escape from cultivation. III. Corn cockle (Agrostemma 

 githago) ; scattered with wheat, frequent in wheat fields. IV. Chicory 

 (Cichorium intybus) ; seed frequently found in alfalfa seed. It has been 

 widely scattered in this way. V. Peppergrass (Lepidium apetalum) ; wide- 

 ly scattered with timothy seed. VI. Parsnip (Pastinaca sativa) ; a fre- 

 quent escape from cultivation. 



(I and V, drawings, Charlotte M. King; II and IV, drawings, Ada Hay den ; 

 III and VI, Hillman.) 



Every phytogeographer is confronted with the problem of placing 

 weeds of the given area in their relation to other floras. Let us 

 take as an illustration a virgin Iowa prairie covered with a close 

 mat of such plants as the blue stems (Andropogon scoparius and 

 A. furcatus), vetch (Vicia americana) , meadow rue (Thalictrum 



