DISTRIBUTION OF DEFENSE NECTARIES IN IPOMOEA (CONVOLVULACEAE)

The structure and distribution of defense nectaries in the genus Ipomoea (Convolvulaceae) were investigated. These nectaries do not reward pollinators and probably contribute to antiherbivore defense. Of 22 species sectioned, 15 had defense nectaries on the sepals. Of 12 other species observed, ten had sepal nectaries .nd two did not. Structurally, 14 ofthe species sectioned had crypt sepal nectaries and one had a basin nectary. Of the 14 species with crypt nectaries, two had invaginated spaces adding greatly to the internal area of these nectaries, and forming the most complex nectaries that have been reported. We term these labyrinthine crypt nectaries. All three types of nectaries are lined with secretory trichomes along the proximal surfaces of the crypts. Species with defense nectaries on the sepals tend to have petiolar defense nectaries as well, but the two locations may have different nectary types; e.g., basins on the petiole and crypts on the sepals. Since most reports of the function of these nectaries have shown antiherbivore defense by nectar feeders, the distributions of defense nectaries with respect to region and life history of the species were sought. Plants without sepal nectaries were found to have significantly smaller seeds than plants with sepal nectaries; they were also more frequently annuals. No significant relationship was found between region or breeding system and defense nectaries. NECTARIES (sugar-water secreting glands) are not confined to flowers but are found on the foliage of plants and on virtually every abovegroundpart(Zimmermann,1932;Napp-Zinn, 1973; Fahn, 1979; Elias, 1983). The function of nectaries within flowers has been shown to be to reward the pollinator and thus to facilitate reproduction. The function of non-floral nectaries (usually termed extrafloral nectaries, but termed defense nectaries by us), appears to be antiherbivore defense: to attract insects which prey on or exclude plants' herbivores (Bentley, 1976, 1977a, b; Deuth, 1977; Keeler, 1977, 1980, 1981 a; Tilman, 1978; Inouye and Taylor, 1979; Koptur, 1979, 1982; Pickett and Clark, 1979; Blom and Clark, 1980; O'Dowd, 1979; Schemske, 1980, 1982; Beckmann and Stucky, 1981; Stephenson, 1982; among others, but see O'Dowd and Catchpole, 1983 and Tempel, 1983, for counter examples). Thus, just as floral nectaries are part of a mutualism of the plant with its pollinators, defense nectaries establish a mutualism of the plant with predaceous insects: the plant provides food in exchange for herbivore removal. Generally, l Received for publication 6 January 1984; revision accepted 30 May 1984. We thank S. Heinisch for his excellent sectioning and W. Glider for technical assistance. This project was supported in part by the University of Nebraska Research Council. We thank D. F. Austin for taxonomic consultation and G. Drohman for growing the plants. ants are the mutualists attracted (Bentley,1976, 1977a, b; Keeler, 1977, 1980, 1981a; Tilman, 1978; Inouye and Taylor,1979; Koptur,1979; Schemske,1980,1982; Beckmann and Stucky, 1981; Stephenson,1982) but other groups may provide the same service, e.g., solitary wasps (parasitoids), predaceous beetles and flies (Koptur, 1982; Hespenheide, unpubl.). The effectiveness reported for the mutualism (refs. above) would lead one to expect that this interaction would be widespread, at least among the members of a particular taxon. Yet, virtually every genus or family in which defense nectaries are known contains species or genera which lack them. For example, in the Passifloraceae and the genus Passiflora, defense nectaries are common on stems and leaves, but P. costaricensis and P. capsularis both lack them (L. E. Gilbert, pers. comm.) as do some races of P. foetida (Baker, Opler and Baker, 1978; Durkee, Baird and Cohen, 1984). Similarly, despite the many defense nectaries in the Fabaceae as a whole, they are unknown in the genera Desmodium (Keeler, unpubl.), and Cassia (sensu Irwin and Barneby, 1982). This study investigates a single genus (Ipomoea) in which defense nectaries are found in two locations: on or close to the sepals and on or close to the distal end ofthe petiole. In some species these nectaries are present in both places, in some only in one and in other species they are entirely absent. The distribution of