Which of these is a slim, cylindrical flower cluster with no petals?
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. 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.