Pollen grains are the carriers of the male gametes or their progenitor cell, in higher plants. They also are important tools for paleoclimatic reconstruction. They reflects the ecology of their parent plants and their habitats and provide a continuous record of their evolutionary history.
Like spores, pollen grains possess a wall highly resistant to microbial attack. This wall comprises two layers, the outer, highly resistant exine mostly composed by sporopollenin, a biopolymer considered “the most resistant organic material known”, and the inner intine that surrounds the cytoplasm. The morphology of pollen grains is diverse. Gymnosperms pollen often is saccate (grains with two or three air sacs attached to the central body), while Angiosperm pollen shows more variation and covers a multitude of combinations of features: they could be in groups of four (tetrads), in pairs (dyads), or single (monads). The individual grains can be inaperturate, or have one or more pores, or slit-like apertures or colpi (monocolpate, tricolpate).
Pollen grains could enter into the fossil record by falling directly into swamps or lakes, or being washed into them or into the rivers and seas. The ones which are not buried in reducing sediments will tend to become oxidized and be destroyed.
They are filtered by differential dispersal in the air and in the water. For instance, large miospores, pollen grains and megaspores will tend to settle out in rivers, estuaries, deltas or shallow shelf areas, whereas small miospores and pollen grains may settle out in outer shelf and oceanic conditions. The differences in pollen productivity and dispersion rates pose a significant problem for palaeoclimatic reconstruction because the relative abundances of pollen grains in a deposit cannot be directly interpreted in terms of species abundance in the study area. Another difficult is that spores and pollen may suffer several cycles of reworking and redeposition, leading to some confusion in the fossil record.
Gunnar Erdtman, a Swedish botanist, published in 1921, his thesis about pollen as a tool for study the Quaternary vegetation and climate change. He was the first to suggest this application for fossil pollen.
Pollen analysis involves the quantitative examination of spores and pollen at successive horizons through a core, specially in lake, marsh or delta sediments, especially in Quaternary sediments where the parent plants are well known. This provide information on regional changes in vegetation through time, and it’s also a valuable tool for archaeologists because gives clues about man’s early environment and his effect upon it.
In a recent study, applied to the Crisis in the Late Bronce Age (LBA) in Cyprus and Syria, the pollen record reveals the presence of plants adapted to drier weather, which indicates a decrease in rainfall. Researchers suggest that this drought lasted about three hundred years causing crop failures, dearth and famine, and forcing regional human migrations at the end of the LBA in the Eastern Mediterranean and southwest Asia.
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