ANT PLANTS

Especially in tropical habitats, plants and ants have evolved tight evolutionary bonds whereby ants provide excellent protection for the plant from herbivorous (phytophagous) animals. Ants living on a plant viciously attack any animal, large or small, that touches the plant or otherwise might do it any harm. Ants can quickly kill insects and small invertebrates, but they can also be extremely irritating to larger vertebrates, causing the intruder to flee. Thus, one can see a powerful selective advantage for a plant to harbor ants, and why, in all likelihood, these coevolutionary relationships became established. In the tropical rain forest, many plants should be avoided by humans, because touching them will cause an attack by the plant-defending ants, and tropical biologists usually carry a mental list of these plants to avoid any confrontations.

[It should also be noted that the high number of ants and ant species that inhabit tree canopies (arboreal ants) has been linked by Diane Davidson and coworkers (University of Utah) to using the plant for a nitrogen source, especially in obtaining nitrogen via exudate of ant-tended homopterans. The paper documenting this phenomenon is in a very useful book published by MEMBG, which is still in stock (domestic sale $15, includes tax and shipping, check only UC Regents).]

There are a number of different plant adaptations that attract ants to their host.

Ants may obtain a useful house on the plant, natural cavities and chambers termed domatia, in which certain ant species can form nests. Domatia are also inhabited by other animals, e.g., mites. A domatium may be of many different forms:

hollow stem internodes, as in species of Cecropia (Family Cecropiaceae) or hollow petioles, as in many tropical legumes
large, hollow or cupped stipular spines, e.g., in bullthorn species of Acacia (Family Mimosaceae)
cavities and tunnels within swollen roots or caudices, such as in myrmecophilous plants that house ants colonies
crypt-like indentations in the leaf surface, e.g., leaves of numerous Rubiaceae, such as tree gardenia (Gardenia thunbergia upper leaf surface, lower leaf surface) and coffee (Coffea arabica), which have natural cavities on the leaf lower surface where a lateral vein diverges from the midvein

A great many plants, from the wet tropics to even dry deserts, have extrafloral nectaries, from which ants obtain sugary sap and thereby remain on the plant to defend it. Having extrafloral nectaries generally detours ants from using the flowers, and thereby interfering with pollination. [Read more about Extrafloral Nectaries]
Müllerian bodies are special food bodies that provide protein to the ant visitors. The most famous example of these occur at the leaf base of Cecropia.
Pearl bodies usually are specialized trichomes (plant hairs) that form on leaves or stems, as the white pearl bodies on leaves of many members of the grape family (Vitaceae) and balsa, Ochroma (Family Bombacaceae).
Special food bodies, called Beltian bodies, occur on the pinnules (leaflets) of many Acacia ant plants. On Macaranga (Family Euphorbiaceae), common pioneer plants pioneer plants in the tropics, food bodies termed Beccariian bodies can occur on leaves and its stipules.
Many seeds are dispersed by ants if the surrounding fruit pulp seed attachment (e.g., aril, caruncle, funiculus, or strophiole) has lipid-containing elaiosomes.

Plants are very important to ants and terms, which may build nests there above the ground, and Atta ants harvest leaves and then farm them within their nests using fungi.

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