As part of an ongoing systematic study of New World Ephedra, 29 taxa were sampled for micromorphological, cytological, and ovule ontogenetic variation. Ephedra grows as dioecious much-branched shrubs, climbers or trees with small opposite or whorled scale- to needle-like leaves in arid regions of the New and Old World. Species are characterized by ovulate strobili with dry membranous, winged or fleshy bracts; lance pyriform to ovoid seeds; microsporangiate strobili with inaperturate, non-saccate, polyplicate pollen; and a base chromosome number x = 7. A combination of newly recognized and reevaluated features of potential taxonomic value includes: micromorphology of stem and seed cuticle, leaf venation, tubillus variation and features of ovule ontogeny. Stomata of Ephedra are haplocheilic, monocyclic and tetracytic and occur in furrows, which alternate with non-stomatiferous bands. Species vary in stomatal opening shape, number of stomatal rows per furrow, epidermal cell shape, position of cuticular flanges, and shape of papillae. Seed coat variation exhibits three distinct types: 1) striate and papillate, 2) striate, and 3) striate and reticulate. Leaf venation patterns may be bi- or trifasciculate. Phyllotaxy determines the number of ovules per strobilus, while a reduction in ovule number has also taken place in some decussate (e.g., E. antisyphilitica) and trimerous (e.g., E. trifurca) taxa. Natural interspecific hybridization and polyploidization are thought to play a role in the evolution of New World ephedras. Cuticular characters confirm the position and parentage of the two interspecific hybrids, E. arenicola and E. intermixta as described by H. C. Cutler. Chromosome numbers are available for 15 species. Of the New World taxa examined, 40 % are diploid, 13.3 % are polyploid and 46.7 % are both diploid and polyploid. These patterns are further tested against O. Stapf's long-standing classification system.

Key words: cytology, Ephedra, micromorphology, New World species, systematics