Article: Megaspores and massulae of Azolla prisca from the Oligocene of the Isle of Wight
Scanning and transmission electron microscopy, ultra-thin sectioning and light microscopy are employed in this investigation of Azolla prisca, which is placed in Section Trisepta sect. nov. of the genus. The columella and structural modifications of the proximal megaspore wall of A. prisca are compared with other species, both fossil and modern, and the phylogenetic interrelationship of these structures are discussed. The complex megaspore wall reveals an exine and two-layered perine, the outer perine layer being further subdivided into four zones. The inner perine layer thickens to form the proximal wall and associated labra. The wall structure of most Azolla species appears to be of this same basic pattern. Massulae, found dispersed and attached to megaspore apparatuses, reveal funnel-shaped cavities connecting the microspores to the exterior. Although previously unrecorded in Azolla such structure is present in both fossil and modern species, and is thought to form an escape mechanism for spermatozoids. A list of the better-known pre-Miocene Azolla species is presented, which includes stratigraphic range, and characteristics expressed as a formula. The evolutionary trends in Azolla are briefly reviewed.