West Cork Palaeoecology

Pollen - small and virtually indestructible

In the pages on this site we delve into the biology, morphology, recognition of pollen, its preparation for examination under the microscope when obtained either as fresh specimens direct from plants, or from current sites of deposition, and also of fossil pollen from sediment.

An understanding of what pollen is, how it forms and how it behaves is a useful insight into identification and recognition of pollen.

The study of pollen is termed palynology, though this can also cover other airborne particles - they are all palynomorphs. Indeed palynology is specifically the study of airborne dust. The general use of the term palynology denotes pollen and spores, however, together termed sporomorphs, and so other palynomorphs are generally termed non-pollen palynomorphs (NPPs). On these pages and throughout this website references to pollen generally also include spores. NPPs are usually named in discussions.

The study of pollen embraces work in the three arenas - the field, the laboratory, and the library.

Pollen is the name given to the microscopically small structures that are used by plants to distribute male sex cells (gametospores) for the purpose of fertilisation. The ultimate goal of a pollen grain is the style of a member of the same species.

Pollen grains are on average generally between 10 microns and 100 microns in diameter - 1 micron is 1/1000th of a millimetre. They are comprised of several outer layers, internal cytoplasm, and two or three cells, at least one of which is the male sex cell that will comprise one half of the new organism after fertilisation. The other cell(s) in the pollen grain provide the growth potential for the pollen tube (see the images on the Prunus page here), which will grow towards the receiving plant's ovary and down which the male sex cell will travel.

We are primarily interested in the outer coatings of the pollen grain, which exists in several layers. The outer layer (generally) is the most durable and is composed of a material called sporopollenin, which is often cited as one of the most durable natural substances known. This fact makes the preservation of pollen grain coatings possible given the right conditions in the deposition environment - a lack of oxygen (anoxic) is the most important. This occurs in wet environments like bogs and lakes, and such environments that are acidic, with a pH below 7, can result in good preservation. Indeed some pollen grains are preserved long enough and well enough to eventually be incorporated into the rock that formed from sediments in which the pollen was deposited. A substantial number of the Devonian and Carboniferous formations mentioned on the Geology pages have been dated to their relative position in the rock record by recognition of miospores - that is spores of 2 microns or less, and thus submicroscopic. These rocks are over 300 million years old.

Different species of plant produce pollen grains of different types, and the shape, size and surface texture of the outer layers of a pollen grain can serve to identify the species of plant from which they came. In some cases identification can only go to genus or even family level. However, overall the analysis of preserved pollen grains, all taken together, can allow us to postulate as to the environment in which they were deposited, and the plants that were growing in and around that place, at that time in the past.

Recent findings suggest that the chemical makeup of sporopollenin can change depending upon certain environmental factors, such as ultraviolet solar radiation. This is clearly a strategy for adaptation, but this fact can be exploited to employ the makeup of the substance from fossil grains as a proxy for those environmental factors.

The photographs of pollen grains represent three conditions of the pollen grains - fresh and untreated, fresh and treated, and acetylated. Fresh and untreated grains are shown as collected. They may be rather dehydrated and shrunken,folded or otherwise distorted, or they may be hydrated and swollen, so largely spherical, or even burst. In a lot of cases the internal contents of the pollen grain (cytoplasm) may extrude through the pores, which can be a useful indicator of the position of the pore. When swollen of course the furrows (colpi) may be stretched and thus become less prominent or even invisible. Some species have a greater coating of oily pollenkitt as a protection against dehydration, bacterial or predator attack, which reflects light and makes them hardly viewable. Others have little or none, or maybe some in patches, round the pore or colpus for example. So when treated, that is, the pollenkitt is washed off, they take on a different appearance. But the process of 'washing' (with alcohol generally) can affect their hydration state and therefore shape. Some have been mounted in glycerine with a fuchsine stain - these are pink to purple; the process required the melting of the glycerine which means the grains get heated - and some swell and burst as they absorb the water based glycerine. Finally, acetolysis is the process used to prepare pollen grains for viewing whether from sediment samples of from fresh. The process removes all organic material except the sporopollenin, thus leaving an empty shell that takes a shape that is not influenced by internal hydrostatic pressure. These should generally appear in their specific shape with surface textures and features visible. Acetylated grains generally appear a pleasant pale yellow colour and both the external surface of the nearside and the internal surface of the farside of the grain are visible at different levels of focus.

Pollen grains from sediment samples are generally stained with safranine, giving a faint yellow to brown stain. Within a sample the degree of staining of different species is often constant, so in each sample some of the species are almost colour coded. For more details on the processes involved see here

Further Reading

• Erdtman G. 1966. Pollen Morphology and Plant Taxonomy. Angiosperms - An Introduction to Palynology 1. Hafner. London, New York.
• Faegri, K. and Iversen, J. 1964. Textbook of Pollen Analysis. Blackwell. Oxford.
• Fraser, W. T., Lomax, B. H., Jardine, P. E., Gosling, W. D. & Sephton, M. A. Pollen and spores as a passive monitor of ultraviolet radiation. Frontiers in Ecology and Evolution 2, doi: 10.3389/fevo.2014.00012 (2014).
• Godwin, H. 1956. The Historyof the British Flora - a factual basis for phytogeography. Cambridge University Press. Cambridge.
• Jardine P. E., Fraser W. T., Lomax B. H., Sephton M. A., Shanahan T.M., MillerC. S., Gosling W. D. Pollen and spores as biologicalrecorders of past ultraviolet irradiance. doi: 10.1038/srep39269 (2016)
• Moore, P.D. and Webb, J.A. 1978. An Illustrated Guide to Pollen Analysis. Hodder and Stoughton. London.
• Stolze, S. and Moenecke, T. 2017. Holocene History of 'Non-Native' Trees in Ireland. Review of Palaeobotany and Palynology. 244. 347–355.
• Svenning, J. and Skov, F. 2007. Could the tree diversity pattern in Europe be generated by postglacial dispersal limitation? Ecology Letters. 10. 453–460.