Plena is one of Puerto Rico’s most enduring strains of music.  Shaped in the 19^th century when immigrants from Barbados merged their rhythmic and sonic ideas with the country’s local sounds, the music became an essential ingredient for parties, parades and celebrations for years to come.  The Afro-Rican music was also used to broadcast the news to migrant workers and deliver political commentary.  By the 1970s salsa and merengue overshadowed this wonderfully rhythmic music, relegating it to “folk” status.  But in 1994, Gary Nunez sought to rekindle the genre and formed the band, Plena Libre (liberated plena).  Ten years later, the seven-piece ensemble – performing at Kennedy Plaza this Friday night as part of Stockton College’s free summer music concert series – has several hits under its belt and has successfully brought plena back into the mainstream in Puerto Rico.  Incorporating traditional instruments with modern touches such as bass and keyboards, Plena Libre has breathed new life into this musical form, helping it evolve into a popular genre once again.  Follow the conga and requinto sounds to Kennedy Plaza Friday night for a sampling of Puerto Rico’s national dance music.  Show starts at 7 pm.