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Contents Summary 483 I. Introduction 483 II. Progress of research on pollen wall development 485 III. The developmental role of the special cell wall 487 IV. Meiosis and the establishment of microspore symmetry 489 V. The origins of the exine during the tetrad stage 490 VI. The free microspore stage to pollen maturation 495 VII. Conclusions 495 Acknowledgements 496 References 496 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. [ABSTRACT FROM AUTHOR]

Morphological observations on pollen grains of Alvaradoa subovata Cronquist (Simaroubaceae) and Roupala brasiliensis Klotzch (Proteaceae) obtained from TEM complement previous studies of Xifreda & Sanso (1998). With the aim of giving details of the morphology and accuracy to the descriptive terminology, exine stratification is described and new observations on pollen grains obtained from SEM are added. [ABSTRACT FROM AUTHOR]