Nectar is the currency used by plants to pay insects and other animals to do their bidding. It costs plants nothing to photosynthesize this sweet medium of exchange using energy from sunlight to rearrange the elements provided by water and carbon dioxide. The recent rise in awareness of the importance of pollinators has highlighted this nectar-based economy. However, flowers are not the only place where nectar serves as coinage in exchange for services.
According to a 2015 University of Florida fact sheet titled, "Many Plants Have Extrafloral Nectaries Helpful to Beneficials" (see "More Information" below), "… few people are aware of the extrafloral nectaries (EFN), nectar-producing glands physically apart from the flower, that have been identified in more than 2000 plant species in more than 64 families."