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In-situ studies of the ultrastructure of the cyanobacteria in the coralloid roots of Encephalartos arenarius, Encephalartos transvenosus and Encephalartos woodii (Cycadales).
Abstract
Cyanobionts (Nostoc spp.) of Encephalartos have the same general ultrastructure as the free-living Nostoc spp. However, differences between the cyanobionts of the different cycad species were observed. The cyanobiont of E. transvenosus lacks a sheath and gas vesicles. Plate-filament arrays are not present in the cyanobiont of E. arenarius but are abundant in the cyanobiont of E. woodii. The peripheral cytoplasm of the cells of the cyanobiont of E. arenarius has a reticulate appearance due to the presence of numerous vacuoles. Despite the apparent degenerate condition of the cyanobionts in the older parts of the coralloid roots, akinetes were not observed. Phycobilisomes also were not observed. Heterotrophic bacteria and fungi were never observed in the cyanobacterial zone but bacteria are present inside the periderm of the coralloid roots of all three cycads. The root cells adjoining the cyanobacterial zone had many of the characteristics of transfer cells.
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