730 



WEED FLORA OF IOWA 



(Franseria discolor), the two annual sunflowers (Helianthus an- 

 nuus, H. petiolaris), small ragweed {Ambrosia art emisiae folia) , 

 buffalo bur (Solanum rostratum), Kocky Mountain bee plant 

 (Cleome serrulata), gumweed (Grindelia squarrosa) and squirrel- 

 tail grass (Hordeum jubatum). 



Fig. 551B I 



Fig. 551B II 



Fig. 551B III 



Fig. 551B IV 



Fig. £51-B. Seeds of immigrant weeds. I. ' Squirreltail (Hordeum jubatum) ; 

 widely scattered with hay and stock trains from the western plains. II. 

 Spinage (Spinacia oleracea) ; a common weed of the Great Basin country. 

 Scattered from cultivated plants. III. Russian Thistle (Salsola kali var. 

 tenuifolia) ; brought to Dakota with flax seed and grain seed; now scat- 

 tered in Iowa with alfalfa seed. IV. Winged Pigweed (Cycloma atri- 

 plicifolium) . 



(II, drawing, L. R. Collins; the others after Hillman.) 



Cosmopolitan Weeds. — A great many weeds, originally, had a 

 very wide distribution, although many of these so-called weeds 

 were probably introduced by man. In many cases it is almost 

 impossible to tell how and whence they came to the places in 

 which they occur. 



Who can trace the immigration of such weeds as common purs- 

 lane, or charlock, or barnyard grass? In the first place, these 



