Pollen Analysis

Pollen is a fine powdery substance, consisting of microscopic grains containing the male gametophyte of gymnosperms (conifers and their relatives) and angiosperms (monocotyledonous and dicotyledonous flowering plants). Pollen is designed for long-distance dispersal from the parent plant, so that fertilization can occur among individuals, in preference to self-fertilization. (However, many species of plants are indeed self-fertile, some of them exclusively so.) Plant spores are another type of reproductive grain intended for dissemination. Plant spores are capable of developing as a new individual, either directly or after fusion with another germinated spore. Among the vascular plants, these types of spores are produced by ferns, horsetails, and club-mosses. However, spores with somewhat simpler functions are also produced by mosses, liverworts, algae, fungi, and other less complex organisms.

Pollen of many plants can be microscopically identified to genus and often to species on the basis of the size, shape, and surface texturing of the grain. In general, spores can only be identified to higher taxonomic orders, such as family or order. This makes pollen, more so than spores, especially useful in typical palynological studies. The integrity of the outer cell wall of both pollen and spores is well maintained under conditions with little physical disturbance and poor in oxygen, and this is why these grains are so well preserved in lake sediment, bog peat, and even the drier deposits of archaeological sites. Fossil pollen has even been collected, and identified, from the teeth and viscera of extinct animals, such as mammoths found frozen in arctic permafrost.

Plant species are not represented in the record of fossil pollen of lake sediments and bog peat in a manner that directly reflects their abundance in the nearby vegetation. For example, plants that are pollinated by insects are rarely detected in the pollen record, because their relatively small production of pollen is not distributed into the environment in a diffuse manner. In contrast, wind-pollinated species are well represented, because these plants emit large quantities of pollen and disseminate it in a broadcast fashion. However, even among wind-pollinated plants, certain species are particularly copious producers of pollen, and these are disproportionately represented in the fossil record, as is the case of herbaceous species of ragweed (for example, Ambrosia artemesiifolia). Among temperate species of trees, pines are notably copious producers of pollen, and it is not unusual to find a distinct, pollen-containing, yellow froth along the edges of lakes and ponds in many areas during the pollen season of pines. Because of the large differences in pollen production among plant species, interpretation of the likely character of local vegetation based on observations of fossil pollen records requires an understanding of pollen production rates by the various species, as well as annual variations in this characteristic.