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These structures derived from a long history of coevolution between the first and third trophic levels. A good example is the extrafloral nectaries which many myrmecophytes and other angiosperms sport on leaves, bracts, stems, and fruits. Nutritionally, extrafloral nectaries are similar to floral nectaries, but differ as they do not encourage the visiting insect to come into contact with pollen at any point. Their existence is therefore not the product of a pollinator-plant mutualism, but rather a tritrophic, defensive interaction.

Plants are able to determine what types of herbivore species are present, and will react differently given the herbivore's traits. If certain defense mechanisms aren't effective, plants may turn to attracting natural enemies of herbivore populations. For example, wild tobacco plants use nicotine, a poison to defend against herbivores. However, when faced with nicotine-tolerant herbivores, they will attract the natural enemies of serious herbivores. As long as the natural enemies have some potential to be omnivorous, plants can provide food resources to encourage their retention and increase the impact they have on herbivore populations. This potential, however, can hinge on a number of the insect's traits. For example, hemipteran predators can use their piercing-sucking mouthparts to make use of leaves, stems, and fruits, but spiders with chelicerae cannot. Still, insects widely considered to be 100% carnivorous have recently been observed to diverge from expected feeding behavior. Some plants simply tolerate a low level of herbivory by natural enemies for the service they provide in ridding the plant of more serious herbivores. Others, however, have structures thought to serve no purpose other than attracting and provisioning natural enemies. These structures derived from a long history of coevolution between the first and third trophic levels. A good example is the extrafloral nectaries which many myrmecophytes and other angiosperms sport on leaves, bracts, stems, and fruits. Nutritionally, extrafloral nectaries are similar to floral nectaries, but differ as they do not encourage the visiting insect to come into contact with pollen at any point. Their existence is therefore not the product of a pollinator-plant mutualism, but rather a tritrophic, defensive interaction.

Human uses[ edit]
Systemic acquired resistance

Plants have shown they have the ability to build resistance to pathogens after initial infection. This ability is called systemic acquired resistance and comes to fruition after a plant has been infected by a pathogen. After surviving infection, the plant contains increased amounts of defense compounds such as chitinases. The increased presence of these defense compounds have shown to help prevent the plant from being affected again. Studies have shown that plants contain the ability to pass this resistance to their progeny. An example of systemic acquired resistance was shown in Arabidopsis thaliana, in which the plants were exposed to fitness reducing levels of the bacterial disease PstDC3000. After exposure, the subsequent 1st generation progeny of the diseased plant exhibited increase resistance to the disease PstDC3000. In this study, while increased resistance for the disease PstDC3000 was found, it is important to note that the progeny had an increased susceptibility to a necrotrophic fungus called Alternaria brassicicola. The progeny also had a decreased response to the hormone jasmonic acid.