A SERIES OF CATKINS
- The top is a fully fertile catkin. It's quite brown and reaching the end of its "life-span".
- The bottom three catkins are male sterile in various stages of aging. The bottom is the freshest of the three, distiguishable by its very green color. In contrast, the second from the top is a male sterile catkin that is almost fully brown.

Compare the first two catkins in this series. Both are similar in age, where the pollinating season is coming to a close. The top is fully fertile, while the second is fully sterile.