Morphology and Distribution of Petiolar Nectaries in Ipomoea(Convolvulaceae)

The distribution of petiolar nectaries in 24 species of Ipomoea was investigated. Petiolar nectaries were found on 12 species (8 new reports, 4 confirmations of previous reports) and quoted from the literature as being found on 3 other species; they were absent from 9 species investigated. The structure of petiolar nectaries in the genus ranges from simple beds of superficial nectar-secreting trichomes (1 species), to slightly recessed "basin nectaries" (8 species), to "crypt nectaries," which are structurally the most complex extrafloral nectaries known (3 species). (Structures were not determined for 3 species.) Petiolar nectaries are present in all subgenera, but all crypt nectaries occur in the same section (Eriospermum). Species with extrafioral nectaries tend to be perennial; species lacking extrafloral nectaries tend to be annual. There is no relationship between temperate or tropical habitat and presence of nectaries. DESPITE recent interest in extrafloral nectaries (EFNs) (Bentley, 1977a; Elias, 1979) there is much that is not known about their taxonomic and ecological distributions. Extrafloral nectaries are nectar-producing glands on a plant that do not function in pollination. A number of studies have suggested that EFNs have an important ecological role: they attract insects which then defend the plant by their activities in repelling or preying on herbivores (Elias and Gelband, 1975; Bentley, 1976, 1977a,b; Keeler, 1977; Schemske, 1978; Tilman, 1978; Inouye and Taylor, 1979; Pickett and Clark, 1979). This important ecological function would suggest that for different species, the selection on this character should change. Thus, EFNs should be rather variable in relatively closely related taxa which are subject to different selective pressures. At present so little is known about the ecology of EFNs that it is difficult to predict how environments select differently on them. It is, however, possible to ask if EFNs are variable within related taxa, and if any correlations with the variation can be detected. Extrafloral nectaries are often present in some members of a genus and absent from others (e.g., Ipomoea, Helianthus, Andropogon), and variation within a species has been noted, e.g., in Ru' Received for publication 28 December 1978; revision accepted 2 May 1979. We wish to thank D. F. Austin for his advice and hospitality to K.H.K., and for verifying the identification of the species studied. We thank him for the use of his unpublished studies. Seeds were kindly provided by A. DerMarderosian, C. R. Gunn, and D. F. Austin. K. W. Lee carried out the SEM studies. G. Drohman has been responsible for maintaining the plants in the face of vigorous insect attack. Partial support (K.H.K.) by NSF SER 77-06931. bus flagellaris (Rosaceae), (Gleason, 1963) and Bixa orellana (Bixaceae) (Bentley, 1977b). Variation within families appears to be the rule (Zimmermann, 1932; Bentley, 1977a; Elias, 1979). The genus Ipomoea (Convolvulaceae) consists of some 500 species (Austin, 1975) with such common representatives as morning glory, moon vine, cypress vine and sweet potato. The species are chiefly tropical, herbaceous, and vining, but there are temperate, woody, and arborescent species. Ipomoea includes species which have and lack EFNs on both the petioles and sepals. Structural variation in the EFNs of Ipomoea has been recognized for a long time (Poulsen, 1877; Gardiner, 1887; Rao, 1930; Zimmermann, 1932; Napp-Zinn, 1973). The EFNs of a few species of Ipomoea have been described morphologically (Poulsen, 1877; Gardiner, 1887; Ewart, 1895), and the ecological function of some species has been investigated (Nieuwenhuis, 1907; van der Pijl, 1954; Keeler, 1977). This paper reports the structure and distribution of petiolar nectaries in 24 New World species of Ipomoea, representing all three subgenera and 7 of the 9 sections of the genus as defined by Austin (1975; pers. comm.). It greatly expands previous work on the distribution of nectary types in the genus and for the first time considers species lacking petiolar nectaries. We further attempt to interpret this distribution in terms of phylogeny and ecology. MATERIALS AND METHODs-Plants of as many species of Ipomoea as possible were assembled from a variety of seed sources. These were grown in the greenhouse at the University of Nebraska, Lincoln, and presence of petiolar nec946 Keeler in American Journal of Botany (September 1979) 66(8). Copyright 1979, Botanical Society of America. Used by permission. September, 1979] KEELER AND KAUL-PETIOLAR NECTARIES IN IPOMOEA 947 TABLE 1. Relationship of presence of petiolar nectaries in Ipomoea to systematics of the genus Petiolar Type of Previous nectary nectary report