Apertures are very small spots on the walls of a pollen

Pollen

Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

, where the wall is thinner and/or softer. For germination it is necessary that the pollen tube

Pollen tube

The pollen tubes is the male gametophyte of seed plants that acts as a conduit to transport the male sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms .After pollination, the pollen tube...

 can reach out from the inner of the pollen and transport the chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s to the egg deep down in the pistil. The apertures are the places where the pollen tube is able to break through the elsewhere very tough pollen wall.

The number and configuration of apertures are often very exactly defined for and characteristic for different groups of plants. The biggest class of plant species, Eudicotyledonae

Eudicots

Eudicots and Eudicotyledons are botanical terms introduced by Doyle & Hotton to refer to a monophyletic group of flowering plants that had been called tricolpates or non-Magnoliid dicots by previous authors...

has three apertures in each pollen.