SYMBIOSIS IN CYCADS - THE ORIGIN AND DEVELOPMENT OF CORALLOID ROOTS IN MACROZAMIA-COMMUNIS (CYCADACEAE)

Coralloid roots of Macrozamia have more apparent developmental stages than those of many other cycad genera, providing an ideal study vehicl e for obtaining a better understanding of the growth and development o f symbiotic cycad coralloid roots. In M. communis L. Johnson, the proc ess begins with initiation of young apogeotropic, papillose roots call ed ''precoralloids'' and involves phases of maturation, cyanobacterial invasion, coralloid formation, senescence, and regeneration. Active p recoralloid apices continue to produce papillose tissue, but during pr ecoralloid maturation, the prominent papillose sheath is gradually rep laced by a thin, dermal layer with scattered lenticels. Cyanobacterial invasion has been observed at different stages of precoralloid matura tion and stimulates further, irreversible development of precoralloids into coralloids. Newly invaded precoralloids in the process of transi tion may be readily identified by their distinctive apical lenticels. Coralloid development involves transformation of the original, apogeot ropic precoralloid tissue as well as production of new coralloid tissu e by apical meristems. Although continuous, these two coralloid region s may be recognized by their external morphology. New coralloid growth involves cessation of papillose sheath production, change in gravitro pic response, proliferation of lenticels, and early differentiation of a conspicuous cyanobacterial zone. Three mechanisms enabling continui ty of the coralloid root system are: 1) production of new precoralloid s and coralloids from bases of existing roots of the same kind; 2) ini tiation of atypical roots from within the internal tissues of degenera ting coralloids; and 3) development of internal secondary periderm dur ing decortication of aging coralloid tissue.