The function of the synergids, two small cells that adjacent to a plant ovule, and which degenerate at about the time that the pollen tubes reach the ovule, are believed to function as guides for the growth of pollen tubes. The synergids seem to regulate the development of the filiform apparatus, which ensures that the pollen tubes grow to the ovule. The degeneration of the synergids releases calcium ions into the area near the ovule, and this is believed to cause the pollen to release the sprem cells so that they can swim down the pollen tube and complete the process of fertilization.

There are a number of recent, technical articles on the synergids and their functioning; you can access one on the synergids themselves here, and one on the calcium release theory here.