For just a few days in the spring, male catkins —- slim clusters of drooping flowers (often crudely described as worm-like strands) drop from the forest canopy. Their female counterparts (usually chunkier and upright) remain in the tree, awaiting a good wind to impregnate them with the pollen from the males. Fertilized female catkins will produce seeds. In an oak tree, they bear acorns.
Around my house, the ground is strewn with male catkins from oaks and silver birch. They are extremely delicate so that any disturbance (such as picking one up to examine it) releases bright yellow pollen. The naked eye cannot see how gorgeous these flowers are but take a look at these enlarged scans!