Ambrosia psilostachya
 
Perennial Ragweed

Pistillate Flower Head, Bottom

Little Quill Lake, near Grid 640
10-Aug-2007

The staminate flower heads form a raceme at the apex of the stem. Below this, the pistillate flower heads are found in clusters in the leaf axils. Each pistillate head consists of a single pistillate floret surrounded by an involucre.  The involucre will eventually harden and form the outer surface of a bur.