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 polle n grains because it is composed mainly of primarily accumulated sporopollen in oil tuft units of plasma membrane glycocalyx origin. In ectexines that e xpand circumferentially the tuft units are separated and the spaces between filled by secondarily, accumulated sporopollenin. The secondarily accumula ted sporopollenin is less resistant to oxidation than that of the primary a ccumulation. The mature ectexine of Calluna pollen. recognized for its high resistance to oxidation in sediments, has tuft units that remain close-pac ked. In the ectexine of Calluna there is no space for secondarily accumulat ed sporopollenin. The ectexine and endexine of Calluna pollen Lire alike in density to electrons and contrast to stains.