The flowers are unisexual and monoecious, the numerous males borne in thick catkins proceeding from the side of last year's shoot.
The stamens become numerous (ten to forty) in the male flowers of a few monoecious genera (Pariana, Luzieta).
Monoecious, and bearing their male flowers in catkins, they are readily distinguished from the rest of the catkin-bearing trees by their peculiar fruit, an acorn or nut, enclosed at the base in a woody cup, formed by the consolidation of numerous involucral bracts developed beneath the fertile flower, simultaneously with a cup-like expansion of the thalamus, to which the bracteal scales are more or less adherent.
Cross-fertilization must of necessity occur when the flowers are structurally unisexual, as in the hazel, in which the male and female flowers are monoecious, or separate on the same plant, and in the willow, in which they are dioecious, or on different plants.
But it is clear that it becomes on this view increasingly difficult to explain the occasional occurrence of tetraspores on male, female and monoecious plants or the role of the carpospores in the life-cycle of Florideae.