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Exine, Pollen, Pollination
Pollen/Pollination
If you were to trace the evolution of pollination mechanisms you would in fact be tracing the evolution of the flower, because the process of pollination is the very raison d'être of the flower. Its sole function is to produce pollen and to exchange it with other flowers, either by launching it onto the wind, or by attracting insects or occasionally a more specialised vector (for example, wild Strelitzia is pollinated by the feet of humming birds). Thus pollen and the process of pollination are the key elements in the reproductive mechanism of almost all flowering plants: it is the method by which they exchange DNA.
Pollen finds its way, by a variety of mechanisms from the anther of its flower of origin to a stigma of either the same, or another flower. There it may 'germinate', i.e. produce a pollen tube which grows down through the style tissue, and through which pass the male gametes. The pollen grain itself is not a sperm cell as such; it contains one or several non-reproductive vegetative cells and one reproductive cell containing two nuclei, one of which divides to form the two sperm cells. These fuse with their female counterparts - the ovules - during fertilisation.
Pollination does not necessarily result in fertilisation. An estimated 70% of plants exhibit self incompatibility, i.e. the means by which the stigma recognises its own pollen and prevents it from germinating - a mechanism to promote outbreeding and limit inbreeding. Self incompatibility mechanisms are the subject of a lot of ongoing research, the details of which lie outside the scope of this note, but usually the mechanisms appear to be genetic, and vary greatly from species to species.
Exine
The exine is the outer coat of the pollen grain. Under the microscope, exines vary greatly and are often deeply sculptured. The shape, size and colour of pollen grains varies enormously and some are sufficiently characteristic to be identifiable with particular plant genera. - see Example.
