Exine

Exine is the outer layer of the pollen grain wall and the spores of seed plants. It is primarily composed of sporopollenin, a chemically inert biopolymer that provides exceptional resistance to decay, desiccation, and chemical attack. The exine surrounds the inner layer, the intine, and is formed during microsporogenesis when the tapetum contributes materials that envelop the developing pollen grain.

Structure and ornamentation: The exine often shows complex surface patterns that are highly species specific. Textures can be reticulate, rugulate, echinate (spiny), or striate, and many pollen grains possess apertures such as pores or furrows called colpi. The arrangement and type of apertures, along with surface sculpture, are key features used in pollen identification in palynology.

Development and components: In many plants, the exine is built from sporopollenin deposited around the developing microspore; its robustness is a major reason pollen and spores can persist in fossils for millions of years. The exine's integrity after the pollen grain’s death makes it especially valuable for studies of ancient vegetation.

Function and significance: The exine protects the male gametophyte during dispersal and helps in adherence and viability. Its patterns aid pollination ecology and may influence interactions with pollinators and the environment. Because it survives after pollen decay, exine morphology is a primary source of data in plant taxonomy, systematics, and paleobotany.