Western Coneflower
(Rudbeckia occidentalis)

These are not sunflowers whose petals have fallen off, as I once thought -- they are coneflowers. This species never has any petals (actually the "petals" would be called ray flowers, if there were any.) They have only the dark brown head, consisting of many tiny perfect disk flowers, at the top of a tall stalk.

Close-up of a coneflower head, showing the rows of disk flowers. Each little spot of yellow is a tiny perfect flower. The ones near the bottom of the cone have already bloomed and faded, while those near the top have not yet bloomed. Around the middle is the ring of flowers that are currently in bloom. (They have pollen that you can see.)

Look at one of these cones with a magnifying glass sometime, to see the details better.