COMPETITION AND ACCOMMODATION BETWEEN APICAL LAYERS AND THEIR DERIVATIVES IN THE ONTOGENY OF CHIMERAL SHOOTS OF PELARGONIUM X HORTORUM

Two mutant plastogenes in all possible chimeral combinations were followed in Pelargonium X hortorum Bailey (geranium) shoots. The part of stem, leaf, or other structure derived from each apical layer was clearly apparent on a cell to cell basis. Shoots typically were composed of derivatives of three apical layers but we found derivatives of as many as four apical layers in some leaves and of five layers in some stems. In chimeras with one of the mutants, Dpl W₁, the amount of tissue derived from the various apical layers was the same, whether the layer was mutant or wild type. We suggest that there are independent apical layers and cell lineages derived from them in nonchimeral shoots, and that their contribution in normal ontogeny is like that of the layers in Dpl W₁ chimeras. In chimeras carrying the second mutant, Dpl W₂, there was much less tissue derived from mutant than from wild-type apical layers. The phenotypic expression of the plastogenes was unchanged by their transmission through male or female gametes. Comparisons of the ontogeny of geranium plants carrying the W₁ or W₂ mutant suggested that, while there was competition between the apical layers and between their derivatives, the genome imposed a definite harmonious interaction or accommodation which led to a final normal morphology of all plant parts and organs through quite different ontogenetic pathways.