A model of exine substructure based on dissection of pollen and spore exines

The exine according to our model, consists of filamentous subunits (∼ 15–40nm in diam.), each containing an axially orientated tubule (∼ 10–15nm in diam.) with numerous lateral branches. Relatively few lateral branches are exposed at the surface of exinous subunits so that most of the polysaccharide‐, protein‐, and lipid‐containing tubular complex embedded within the sporopolleninous subunit is protected from degradation and unavailable to stains or ions that could cause molecules of the complex to swell or contract. In the intact exine, the few branches of the tubular complex (glycocalyx units) that are exposed at surfaces of subunits may affect exine size and staining, but their effect is not discernible from exogenous and nonstructural endogenous substances in the microcapillary space between exinous subunits. The existence of glycocalyx units is evident following treatment that etches the sporopollenin from exinous subunits (and eliminates exogenous and endogenous substances), without inactivating glycocalyx units exposed by the removal of sporopollenin. After sporopollenin is partially removed the exine remnant is osmophilic, filamentous in appearance, and the exposed glycocalyx units stain intensely for acidic polysaccharides and moderately for protein. The polyacid groups in exposed glycocalyx units expand when ionized in hydroxide solutions and force exinous subunits to separate, increasing the size of the exine and its reaction to stains but decreasing resistance to degradation. In acid solutions and dehydrating solvents the glycocalyx units contract. Sporopollenin contracts elastically with reduction of microcapillary spaces; exine size and stainability are minimal and resistance to degradation is increased. Theoretically, the glycocalyx units embedded in sporopollenin and protected from degradation even in some sediments of great age are specific in composition and can be used to aid in delimiting past as well as present plant relationships.