What are Spores and Pollen Grains?
Spores, as referred to here, include the reproductive bodies of lower vascular plants such as club mosses, horsetails, and ferns. The earliest occurrences of spores apparently produced by land plants in the fossil record are in Lower Silurian rocks, slightly preceding the appearance of the the first vascular plant megafossils (Cooksonia). Pollen grains are the sperm-carrying reproductive bodies of seed plants, including gymnosperms (such as conifers and cycads) and angiosperms (the flowering plants). Pollen first occurs in the Upper Devonian rocks, corresponding to the occurrence of the earliest fossils seeds (Archeosperma) in North America. Both spores and pollen have very resistant walls composed of a substance known as sporopollenin, and the resistance and inert nature of this wall allows preservation of pollen and spores in sediments under a variety of conditions. Pollen and spore walls of each plant species bear certain types of sculpture and have characteristic apertures; these f
