636 



WEED FLORA OF IOWA 



Fig. 500A Pig. 500B 



Fig. 500. A. Weed seeds eaten by birds, a. Wild buckwheat (.Polygonum con- 

 volvulus) ; b and d. Amaranth or Pigrweed ; c, Chickweed ; e, Spotted 

 Spurge ; f , Ragweed ; g. Foxtail ; h. Dandelion. 



B. Seeds of sedges carried by water. 

 (A, after Dewey, U. S. Dept. of Agr. ; B, drawing by Ada Hayden.) 



tumble weeds among which are several mustards like tumbling 

 mustard. The seeds of the sedge family (Cyperaceae) are com- 

 monly surrounded by inflated sacs or bear tubercles which enable 

 them to be carried by water. Grasses (Gramineae) show consider- 

 able variation in means of dissemination. Some grasses, for ex- 

 ample Stipa, have sharp fruits with twisted awns a prominent 

 factor. While dissemination is a prominent factor in the distribu- 

 tion of plants, the factor of adaptation to habitat is no less im- 

 portant for if a plant cannot adapt itself to the conditions in the 

 habitat where the seed falls the transportation is of no avail so far 

 as its development is concerned. Canada thistle produces some 

 seed in northern Iowa, where habitat conditions are inore favorable 

 than in southern Iowa. 



