Detail of a male flowering catkin on a willow
A catkin or ament is a slim, cylindrical flower cluster (a spike), with inconspicuous or no petals, usually wind-pollinated (anemophilous) but sometimes insect-pollinated (as in Salix). 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. For some time, they were believed to be a key synapomorphy among the proposed Hamamelididae, also known as Amentiferae (i.e., literally plants bearing aments). Based on molecular phylogeny work, it is now believed that Hamamelididae is a polyphyletic group. This suggests that the catkin flower arrangement has arisen at least twice independently by convergent evolution, in Fagales and in Salicaceae. Such a convergent evolution raises questions about what the ancestral inflorescence characters might be and how catkins did evolve in these two lineages.
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 (Comptonia).
The word catkin is a loanword from the Middle Dutch katteken, meaning "kitten" (compare also German Kätzchen). This name is due either to the resemblance of the lengthy sorts of catkins to a kitten's tail, or to the fine fur found on some catkins. Ament is from the Latin amentum, meaning "thong" or "strap".