730 



WEED FLORA OF IOWA 



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

 nuus, H. petiolaris) , small ragweed (Ambrosia artemisiaefoUa) , 

 buffalo bur {Solanum rostratum) , Eocky Mountain bee plant 

 (Cleome serrulata), gumweed {Grindelia squa/rrosa) and squirrel- 

 tail grass (Hordeium jubatum). 



Pig. 551B I 



Fig. 551B 11 



Fig. 651B III 



rig..551B IV 



Fig. 551-B. Seeds of Immigrant weeds. I. Squirreltail {Hordev/m jubatum') ; 

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

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

 Scattered from cultivated plants. III. Russian Thistle iSalaola kali var. 

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

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

 plicifolmm'). 



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



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

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

 were probably introduced by man. In many eases 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 



