Flatbud Pricklypoppy-Argemone munita

This long-blooming white flower with the papery thin flower petals and the prickly stems and leaves is flatbud pricklypoppy (Argemone munita), the only member of the Poppy Family (Papaveraceae) in this area. It is native to the western United States and grows often in disturbed areas, such as along roadsides.

 
Nestled among the white petals is a yellow clump of 150-200 stamens, with a maroon stigma appearing right in the middle. 

The flower buds are also prickly.

Here's a closeup of some of those prickles on the stems and leaves. They deter animals from eating this plant.

For more information about flatbud pricklypoppy, click here.