Catkin

Catkin-bearing plants include many trees or shrubs such as birch, willow, hickory, sweet chestnut, and sweetfern (Comptonia).

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.

In Britain, they can be seen in January or February, when many trees are bare for winter. They can even occur in December.[1]

For some time, catkins 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.[2][3] This suggests that the catkin flower arrangement has arisen at least twice independently by convergent evolution, in Fagales and in Salicaceae.[4] Such a convergent evolution raises questions about what the ancestral inflorescence characters might be and how catkins did evolve in these two lineages.

Etymology illustrated by pussy willow catkins from a children's book

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.[5][6] Ament is from the Latin amentum, meaning "thong" or "strap".[7]

• Catkins in Fagales
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  Male catkins of hop-hornbeam (Ostrya carpinifolia)
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  Young male catkins of hazel (Corylus avellana)
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  Mature male catkins of hazel (Corylus avellana)
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  Alder female catkins (Alnus sp.)

• Catkins in Salicaceae
• 
  Young male catkin of a willow (Salix sp.)
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  Three male catkins on a willow (Salix sp.)
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  Female flowering catkin on a willow (Salix sp.)