Exine, also called exospore, is the durable exterior shell of a pollen spore, providing resistance against decay, dehydration, and other forms of damage. It is made of sporopollenin, one of the most chemically inert biological polymers yet discovered. The exine is so durable that intact pollen spores have been recovered as fossils, and are even common in some types of sedimentary rock.
The exine encloses the intine, a much softer secondary protective layer, which itself encloses the vegetative cell and generative cell of the pollen spore.
The exine usually has some kind of surface area texture which better enables it to adhere to surfaces or float aerodynamically on a breeze, facilitating pollen transmission over distance. These formations - usually spines, wart-like bumps, or depressions in the surface - are useful for identifying individual plant species, and in apiculture (beekeeping) this identification helps track how far afield bees fly for food.
Exine spines are typically less than a micron in length, with spores bearing this feature called spinulose spores, but some exceed a micron in length and are called echinate spores. Spores with net-like patterns are reticulate spores.
Iron Noder 2018, 4/30