Spores are produced as part of their reproductive cycle and can be thought of as a type of primitive seed. Pollen-producing plants first appeared in the late Devonian Period, about 360 million years ago, and have become the dominant plant type over most of the earth. Because these plants have become increasingly abundant and diverse from the Devonian Period to the present, pollen grains have become more dominant in the geologic record and spores have consequently decreased in abundance. In most modern samples, for instance, pollen grains generally form more than 95 per cent of the sample. Many coniferous and flowering plants produce vast quantities of pollen as part of their reproductive cycle.
Pollen grains carry the male genetic information for fertilization.
From this, seeds are formed which ultimately can produce new plants. Pollen grains are dispersed widely over the landscape, mainly by wind, and are a major cause of the hay fever and allergies suffered by many people in the summer months. Plants produce many more pollen grains than are actually used in reproduction. Much of the pollen falls to the ground or is washed from the atmosphere by rain and ultimately becomes part of sediments, especially the mud accumulating on the floor of lakes. Because pollen grains are so tiny, less than a teaspoonful of sediment can contain many thousands of them.