ON THE PHYCOBIONTS OF THE CYCAD CORALLOID ROOTS

S ummary The structure, ultrastructure and the developmental cycle of the phycobionts living in the coralloid roots of various cycads growing in different Italian Botanical Gardens were studied. The phycobionts were examined in the various zones of the coralloids from the top towards the base. All the phycobionts observed have the characteristics typical of Nostoc. Eucaryotic algae, fungi or bacteria have never been observed. Nostoc present in the coralloid roots of various cycads differ from each other in the trichomes and the organization of vegetative cells and heterocysts. Three main types of phycobionts have been distinguished, each having preference for a group or species of cycad. In the coralloid roots the heterocystous phase prevails with vegetative cells and heterocysts. The latter differ from the heterocysts reported in other cyano‐bacteria. Their frequency increases towards the basal parts of the coralloid roots and concomitantly they undergo changes until they become empty and stiff. Heterocysts releasing gonidia or dividing were not observed. In the sporogenous phase akinetes were not found. In the oldest coralloid roots few vegetative cells and numerous heterocysts in various stages were found.