A catkin or ament is a slim, cylindrical flower cluster with inconspicuous or no petals, usually wind-pollinated but sometimes insect pollinated. They contain many, usually unisexual flowers, arranged closely along a central stem which is often drooping. They are found in many plant families, including Betulaceae, Fagaceae, Moraceae, and Salicaceae.

In many of these plants only the male flowers form catkins, and the female flowers are single (hazel, oak), a cone (alder) or other types (mulberry). In other plants (such as poplar) both male and female flowers are borne in catkins.

Catkin-bearing plants include many other trees or shrubs such as birch, willow, hickory, sweet chestnut and sweetfern, and also some herbaceous plants such as nettle.

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