Question

Passage:
In 1916, Swedish geologist Lennart von Post introduced pollen analysis, or palynology, as a quantitative method to reconstruct quaternary paleoenvironments. Prior to von Post’s innovation, paleobotanists relied primarily on macrofossils—such as preserved leaves, seeds, and wood fragments—to infer historical plant distribution. However, macrofossils offered an incomplete record because their preservation depended on fortuitous depositional events, leading to spatial bias toward low-lying lacustrine basins. Von Post recognized that pollen grains possessed outer walls composed of sporopollenin, an extremely durable biopolymer capable of surviving in acidic peat bogs for millennia without structural decay. By extracting sediment cores from peat deposits and systematically counting microscopic pollen grains at uniform vertical intervals, von Post established relative frequency diagrams that tracked shifts in dominant forest canopy species over time. Crucially, von Post noted that because wind-pollinated trees produce vast quantities of buoyant, airborne pollen, regional vegetation signals are averaged over considerable geographic radii, mitigating the hyper-local bias inherent in macrofossil sampling. Nevertheless, von Post explicitly cautioned that differential pollen production rates among species—such as the prolific dispersal of pine pollen compared to the sparse shedding of insect-pollinated taxa—necessitated the application of empirical calibration factors before pollen proportions could be directly equated to absolute forest composition.

According to the passage, Lennart von Post identified which of the following as a specific property of pollen grains that enables their long-term preservation?