A CONTRIBUTION TO THE STUDY OF POLLEN WALL ULTRASTRUCTURE OF ORCHID POLLINIA^ Michael S. Zavada^ Abstract Pollen grains of 30 orchid species from the Spiranthoideae and Epidendroideae (including the Vandeae) were studied ultrastructurally. This study complements the published palynological literature on the Apostasioideae, Cypripedioideae, Spiranthoideae, and Orchidoideae. The orchid pollen thus far studied exhibits a wide range of variation in pollen unit, aperture type, and wall ultrastructure. The least specialized Apostasioideae and some Cypripedioideae shed pollen in monads and have monosulcate grains with a tectate-columellate, perforate exine structure. These are features observed in most primitive monocotyledons. In the more specialized orchid subfamilies, pollen occurs in mealy or tightly packed, waxy pollinia, and the grains have a porate-ulcerate aperture or are Inaperturate, and the sporopolleninous wall is present only on the most peripheral grains in the pollinium (a few species lack exines). The presence of the exine on only the most peripheral grains in the pollinium is accompanied by a loss of the footlayer in many taxa and elaboration of the intine. As in other monocots, endexine has not been unequivocally demonstrated in orchids. The Orchidaceae comprise about 1,000 genera ily Spiranthoideae have been investigated palyno- and 15,000-20,000 species. Dressier (1981, 1983) logically to some extent (Balogh, 1979; Williams has separated the orchids into five subfamilies: & Broome, 1976). Pollen grains are shed in loose Apostasioideae, Cypripedioideae, Spiranthoideae, pollinia and are generally inaperturate. Exine Orchidoideae, and Epidendroideae (including the sculpturing is reticulate (especially in the Spiran- Vandeae). thinae), and wall structure varies depending on the There is general agreement among taxonomists position of the pollen grain in the pollinium: pe- that the Apostasioideae are the least specialized ripheral grains are usually tectate-columellate with (primitive) subfamily of the Orchidaceae (Dressier a thick footlayer; those in the interior of the pol- & Dodson, 1960; Dressier, 1981, 1983), with two linium usually lack the tectum, with only the col- genera and about 16 species, found primarily in umellae and footlayer being present, tropical Asia. Pollen morphology and ultrastructure Pollen of the Spiranthoideae and Epidendroideae have been investigated (Schill, 1978; Newton & have been httle studied using transmission electron Williams, 1978) and the pollen shows features of microscopy (however, see examples in Chardard, many other monocots (Zavada, 1983). The grains 1958, 1969; Heslop-Harrison, 1968; Dulieu 1973; of the Apostasioideae are usually shed in monads Gaspers & Gaspers, 1976; Balogh, 1982; Balogh and are monosulcate. Exine sculpturing is reticu- & Mann, 1982; Hesse & Burns-Balogh, 1984; late. Pollen wall structure is tectate-columellate Wolter & Schill, 1986). The present contribution with a thin footlayer and no endexine. describes the pollen wall structure of 30 taxa from The Gypripedioideae, primarily a tropical group, these subfamilies (only one taxon from the Spiran- have four genera and about 130 species. This thoideae). subfamily Ukewise exhibits many primitive features 1981). Pollen is Will Methods generally monosulcate or ulcerate (i.e., with an ill- defined pore). Exine sculpturing can be reticulate Pollinia were removed from living material and or verrucate to scabrate. The pollen wall infra- placed immediately in cacodylate HCl-buffered glu- structure, as revealed by SEM, appears to be col- taraldehyde-formaldehyde, followed by fixation in osmium tetroxide, dehydration in an ethanol series, and embedding in Dow Epoxy Resin 334. Pollinia were sectioned on an LKB-1 ultramicrotome, se- W fer, 1977). Representative taxa of the rather large subfam- ' I thank Tim Reeves and Eric Christenson for providing the live material used in this study. - Department of Biology, The University of Southwestern Louisiana, Lafayette, Louisiana 70506, U.S.A. Ann, Missouri Bot. Card. 77: 785-801. 1990