As any sperm can tell you, fertilizing a mate is an arduous task. Animal sperm need to be champion swimmers. Pollen must be aerial acrobats.
Plants are the architects of life on land. It’s a dangerous job. They need to protect themselves from pesky animals, as well as coopt them when they can.
Woody plants shield themselves with bark. Their cork has an intricate structure, providing a guard while allowing interaction with the environment.
Spores and pollen have to either fly through the air on their own accord, hoping to land on a mate, or hitchhike on a pollinator. Either way, abrasion awaits. So pollen need protection.
Millimeter-scale cellular foams are found in cork, bone, and sponge. They provide a confluence of superior insulation, strength, and elasticity, which varies by adaptive purpose.
Likewise, pollen are protected by a fortress. The outer wall is a solid polymer nanofoam, with micrometer cavities just large enough for a pollen to conduct its business. Whereas cork sits still, pollen foam is for a suitor on the roam.
Reference:
Ruxandra Cojocaru et al, “A biological nanofoam: The wall of coniferous bisaccate pollen,” Science Advances (9 February 2022).