Earliest floral evidence for the Ebenaceae in Australia
Flowers are seldom preserved in an identifiable condition. The limited knowledge of floral structure of ancient flowering plants makes comparisons with living plants difficult, as floral structure is the basis for classification of the angiosperms. When found, fossil flowers contribute considerably to our understanding of the evolution of flowering plants1–6. The flowers described here were recovered from Upper Eocene deposits near Anglesea, Australia. They are tetramerous, unisexual, male, with 16 stamens and tricolporate pollen. These flowers, and associated ebenaceous leaves, are the oldest known remains of the Ebenaceae from Australia.
Two new species of Bowenia Hook, ex Hook, f. from the Eocene of eastern Australia
Two new species of Bowenia Hook. ex Hook, f. (B. papillosa Hill and B. eocenica Hill) represent the first report of this genus in the fossil record. Cuticular and architectural differences support their new specific status; however, the overall morphological similarity of the two fossil species to extant Bowenia suggests that evolution in this genus is slow. The fossil localities illustrate that in the Eocene Bowenia occurred further south than it does at present. Physiognomic analyses of the fossil dicotyle-donous leaves at the two localities indicate a warm-temperate to subtropical climate. It is therefore likely that the fossil species lived in environments similar to that of extant B. spectabilis Hook. ex Hook, f.
THE EVOLUTION OF POLLEN TETRADS IN ONAGRACEAE.
In Onagraceae, pollen is shed in mature tetrads in most Epilobieae, many species of Ludwigia (Jussiaeeae), and two closely related species of the large genus Camissonia (Onagreae). Mature tetrads of Camissonia cardiophylla and representative species of Epilobium and Ludwigia were examined with light, scanning, and transmission electron microscopes. Morphological diagnoses of monad units indicated that individual taxa could be readily distinguished. Statistical analyses of tetrads which remained after acetolysis treatment revealed significant differences in the strength of the binding mechanisms. Mechanisms of tetrad cohesion were found to consist of two principal types. Common to all taxa is cohesion of pollen wall surfaces at the aperture margins; this mechanism is well known in many angiosperm groups. With the exception of Camissonia, the remaining taxa also display binding by means of short exine fragments between adjacent pollen units. These fragments, termed bridges and reported here for the first time, are located in the area extending from the aperture margins to near the center of the proximal exine faces. Thin sections reveal that layers of the bridges are identical with those of the exine. Comparisons were made between bridges and viscin threads, both of which occur on the proximal faces of the grains. Viscin threads are present on all pollen grains in Onagraceae and exhibit distinctive morphologies, and bridges were viewed morphogenetically as related to viscin threads but including an endexine layer and occupying a position near the apertures where cohesion of wall surfaces also occurs. In an evolutionary sense, the formation of mature tetrads almost certainly occurred independently in Camissonia and may have done so in Ludwigia and the Epilobieae.