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Gymnosperms and Angiosperms
Plants can be classified into one of two types according to their reproductive methods: gymnosperms and angiosperms. Gymnosperms -- believed to be an older branch of plants that includes conifers, ferns and mosses -- have male cones that release pollen into the air that is captured by female cones before fertilization occurs. This is where the advent of sap comes from, as it is used to trap airborne pollen. Angiosperms are flowering plants that send pollen from the male reproductive organ, the stamen, to the female reproductive organ, the pistil. This can be facilitated by broadcasting pollen into the wind or by enlisting the help of insects and animals, willing or otherwise. By using insects to transport pollen, plants can use less energy in pollen production since its distribution is more focused.
General Co-evolution
The generic case of co-evolution in terms of pollination is demonstrated in the fact that some plants and insects have not evolved specifically for the benefit of just one species. This means that some plants have evolved to have their pollen spread by bees, beetles and other insects rather than just one species. This type of pollination results in wasted pollen and therefore wasted energy and resources due to the fact that many of these pollinators do not visit the same type of plant very often.
Specific Co-evolution
When insects and plants evolve together, each succumbing to the forces of natural selection to whittle away characteristics that do not match up with the other, this is called specific co-evolution. For example, many flowering plants have distinguishing markings that attract only a certain type of pollinator. Other plants and insects have evolved further to change their shapes to accommodate each other, such as the yucca plant and the yucca moth. The opening of this plant's flower is sized just right for this type of moth to crawl into to become covered in pollen as it feeds. The common snapdragon plant is another example, evolving so that its flower's shape is sized exactly to a bumblebee's shape, according to the University of Cincinnati.
Flower Shapes
While flowers are the first thing that attracts pollinators to a plant and must be accommodating to insects, the plants must also protect themselves from insects. An example of this type of protection can be seen in the folded-over leaf structure of some plants so that the ovule would not be eaten by hungry insects. Other plants, such as the tongue orchid, have evolved to look exactly like a female wasp to attract male wasps. When the duped wasp begins having sex with what he thinks is a female wasp, the orchid then covers the wasp with pollen, which then gets spread to other plants.