GARSIDE RULE AND THE MICROSPORE TETRADS OF GREVILLEA-ROSMARINIFOLIA A-CUNNINGHAM AND DRYANDRA-POLYCEPHALA BENTHAM (PROTEACEAE)

Tetrad formation in triporate pollen of Grevillea rosmarinifolia and biporate pollen of Dryandra polycephala were observed with differential interference contrast microscopy. The former species was also studied in the SEM. In Grevillea cytokinesis is simultaneous and the tetrads are tetrahedral with the apertures developing in groups of three at four points within the tetrad, conforming to Garside's rule. In Dryandra cytokinesis is successive with a distinct dyad stage, following which decussate or tetrahedral tetrads are formed. In both species, cytokinesis involves centrifugal cell plates. The microspores remain in contact at their apertures throughout the tetrad stage while their proximal surfaces are separated by the continued deposition of the special cell wall. Grevillea microspores are held in loose tetrads until at least the vacuolate microspore stage by a fibrillar layer of the special cell wall. The occurrence and systematic significance of Garside's rule is discussed and the phenomenon is interpreted as a potential synapomorphy of the Proteaceae and the Olacaceae.