Why the endexine and ectexine differ in resistance to oxidation. Calluna as a model system

The endexine is more resistant to oxidation than the ectexine of most pollen grains because it is composed mainly of primarily accumulated sporopollenin on tuft units of plasma membrane glycocalyx origin. In ectexines that expand circumferentially the tuft units are separated and the spaces between filled by secondarily accumulated sporopollenin. The secondarily accumulated sporopollenin is less resistant to oxidation than that of the primary accumulation. The mature ectexine of Calluna pollen, recognized for its high resistance to oxidation in sediments, has tuft units that remain close-packed. In the ectexine of Calluna there is no space for secondarily accumulated sporopollenin. The ectexine and endexine of Calluna pollen are alike in density to electrons and contrast to stains.