A Contribution to the Study of Pollen Wall Ultrastructure of Orchid Pollinia

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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.

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