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Abstract GRILLI CAIOLA, M. G. (University of Rome, via E. Carnevale, 00173 Rome, Italy). Structural arrangement of phycobionts in coralloid roots of cycads. Memoirs of The New York Botanical Garden 57: 94"103. 1990. The cycad coralloid root organization has been studied by light and electron microscopy. Gonidial zone sections taken at different levels from the apex to the base were treated by different cytochemical reactions to demonstrate the nature of the components of the mucilage filling the intercellular gonidial layer. The gonidial zone of the coralloids from various cycad species shows a similar structural organization. Phycobionts consist of vegetative cells, heterocysts and akinetes. These latter are abundant in the coralloids cultured for a year at 25C in the various mineral nutritional media. In the old parts of the coralloids the heterocyst ultrastructure seems to indicate that the heterocysts could divide, originating new vegetative cells or colonies.