Contents Summary 483 Introduction 483 Progress of research on pollen wall development 485 The developmental role of the special cell wall 487 Meiosis and the establishment of microspore symmetry 489 The origins of the exine during the tetrad stage 490 The free microspore stage to pollen maturation 495 Conclusions 495 Acknowledgements 496 References 496 Summary The outer pollen wall, or exine, is more structurally complex than any other plant cell wall, comprising several distinct layers, each with its own organizational pattern. Since elucidation of the basic events of pollen wall ontogeny using electron microscopy in the 1970s, knowledge of their developmental genetics has increased enormously. However, self‐assembly processes that are not under direct genetic control also play an important role in pollen wall patterning. This review integrates ultrastructural and developmental findings with recent models for self‐assembly in an attempt to understand the origins of the morphological complexity and diversity that underpin the science of palynology.

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We wish to acknowledge the technical assistance of J. Bittner at the Smithsonian Institution and W. Chissoe at the University of Oklahoma. The scanning electron micrographs were prepared at the Scanning Electron Microscope Laboratory at the Smithsonian Institution. We thank Stanwyn â-Shetler for his critical review and helpful comments. We are particularly grateful to Beth Lawler for her assistance in preparation of the manuscript.

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Our results and those we review indicate that the exine has a great capacity for modifications that enable nutrients to pass through from the anther loculus to the microspore cytoplasm. Avenues of passage include strands, some of which are viscin threads, from the tapetum to microspores in, for example, Betula, Fuchsia, and Epilobium. Micro channels in Lopezia, Gaura, and Gelsemium extend through the ectexine, endexine, and intine to the cytoplasm. The bulge regions in Epilobium represent portions of the endexine that become very greatly enlarged, forming conducting channels that transport materials into the microspore cytoplasm. Results with tracers such as colloidal iron and lanthanum have also shown that exines of microspores are permeable across areas lacking obvious channels.Key words: Betula, Epilobium, exine, Fuchsia, Gaura, Gelsemium, Lopezia, microchannels, pollen, tapetum, tufts, viscin threads.

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