Sugary Exudates in Plant Pollination

Sugary secretions are present in many plants and frequently they are linked with reproductive processes. Most of the gymnosperms, both extant and extinct, possess a pollination drop, a diluted sugary secretion protruding from the micropyle, which serves for pollen capture, hydration, and transport in the ovule. It is most probable that this secretion attracted insects giving origin to a plant–insect relationship for pollination based on a sugary solution well before the raise of angiosperm. Floral nectar, a new type of sugary exudate produced by a specific secreting tissue (the nectary), evolved rapidly when the transition from naked ovule to closed carpel was completed and the pollination drops were no longer available as a food resource for insects. Floral nectar is widely distributed and very diverse in the extant angiosperms where it represents the more common reward for a large variety of pollinators. In this chapter, we highlight the evolutionary relationship between nectar and pollination drops in terms of morphology, physiology, ecology, and biochemistry.