Xanthium strumarium
 
Cocklebur

Pistillate Flower Head

Valeport Recreation Site, shore of Last Mountain Lake
18-August-2016

Note the two florets (F1 and F2), each of which contains two exserted style branches.  The proximal involucral bracts have hooked tips but there are a few horn-like distal bracts which are larger and have less hooked tips than the proximal ones.

There are two types of flower heads.  Staminate flower heads contain a number of staminate florets, each consisting of a column of united filaments terminated by five distinct anthers.  Pistillate flower heads contain two pistillate florets surrounded by an involucre of persistent bracts with hooked tips.  Each ovary occupies one of two chambers in the interior of the involucre and each ovary is terminated by a two-branched style which protrudes beyond the involucre.  The involucre will eventually harden and become a burr which contains the two achenes.