Several investigations have focused on mature pollen of Ephedra; however, little is known about pollen ontogeny. This article is the first to describe the complete pollen developmental sequence in the genus. Combined light, scanning electron, and transmission electron microscopy were used to document all major developmental stages in Ephedra americana. Microspore mother cells are characterized by an unevenly thickened callose envelope. Both tetrahedral and tetragonal tetrads occur, but the

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... ity of tetrads exhibit tetrahedral geometry. Pollen wall development is initiated in regions that will become characteristic plicae. The majority of exine deposition, including formation of the tectum, infratectum, foot layer, and endexine, occurs throughout the tetrad stage, although endexine deposition continues into the free microspore stage. The intine forms during the late free microspore stage. Two types of infratectal elements occur. Granular infratectal elements are the predominant type and develop throughout ontogeny, whereas columellar elements form in the early tetrad stage but subsequently become indiscernible. Mature grains are elliptic, have a series of longitudinal plicae, and are inaperturate, yet the exine is very thin in the furrow regions between plicae. Pollen grains with straight and undulated furrows co-occur in the same pollen sac, with the straight morphology dominating. Because several key characters are revealed only during an investigation of the full ontogenetic sequence, this study emphasizes the importance of integrating pollen developmental characters into phylogenetic analyses.