Picture of an Apia
pollen grain, from the carrot family (left) and a Tilia, or basswood, pollen
pollen grain is perhaps the most astonishing. These grains not only have three
colpi, but also a central pore in each colpus. In this case the pore provides
the emerging pollen tube a point of germination during pollination. Variations
in the structure of the surface of the grain in addition to pore features of
tricolporate grains are useful taxonomic elements. Tricolporate pollen grains
can be found in the fossil record from the Upper Cretaceous to recent.