614 



WEED FLORA OF IOWA 



The flower axis is so much shortened in the dandelion, sunflower 

 and composites in general, that it may be a concave, flat, or conical 

 surface. The flowers are crowded upon this surface, and the en- 

 tire group is surrounded by one or more rows of leaf-like bracts 

 which form the involucre. This is the close head or composite 

 type of inflorescence. 



\ 



Pig. 474. Milkweed with flowers in umbels. 



In many eases, as in field sorrel and in five-finger {Potentilla 

 norvegica), the oldest flower of the floral axis is terminal and all 

 later flowers must arise from axils below. This is the determinate 

 type of inflorescence and this type of flower cluster is called a cyme. 



