C is for catkin, it is a dense, cylindrical drooping flower (without petals) in some trees such as willows, oaks, or birches. Catkins are either all male on one tree or all female on another tree. Male catkins are about 1 inch long while female catkins are less than one inch long. I think the catkins pictured here are male. Catkins are usually pollinated by the wind or sometimes by insects.
Here is my picture of the catkins on the weeping willow in my yard. I think they must be male catkins. I took the photo March 19 of this year.