Catkin is a tassel like flower cluster that consists of numerous small flowers arranged around a long central axis. Each flower lacks the colored petals that are typical of most familiar wild flowers. For this reason, the grayish- or yellowish-green catkins are rarely thought of as flowers. Each catkin is either staminate (male, producing pollen) or pistillate (female, producing seeds). The wind pollinates the female catkins. The most familiar catkin is the pussy willow. Catkins appear on the branches of willows, alders, and poplars long before other spring flowers appear. Most catkins are long and drooping. They are also called aments. 

Contributor: Richard A. Jaynes, Ph.D., Horticulturalist and Consultant, Broken Arrow Nursery.