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Chrysanthemum morifolium, explants, mutation breeding, adventitious bud technique, chimeral and nonchimeral mutants, in vivo, in vitro, X-rays, solid mutants. SUMMARYDuring experiments, which are being carried out to study the factors which control the process of adventitious bud formation in vivo on detached leaves of Chrysanthemum morifolium RAM, adventitious shoots were produced from leaves, irradiated with 500 rad of X-rays. The most important but disadvantageous result was that the majority of the adventitious shoots proved to be of a chimeral nature and obviously developed from more than one cell.An in vitro adventitious bud technique was developed using different types of explants. Pedicel segments regenerated the highest number of adventitious shoots and, moreover, they developed faster as compared to explants of young flower heads or leaves. The mutants produced by irradiating the various explants were almost exclusively of a solid (non-chimeral) nature. In addition, histological observations suggest that single epidermal cells are involved in the initiation of the adventitious shoot apices.The optimum dose for mutant production is approximately 800 rad X-rays. Rather often, more than one phenotypically identical mutant was found, which was always derived from the same explant. They could for instance originate from a multi-apical meristem formed by a single mutated cell.

Since a mutation is a unicellular event, irradiation of a multicellular meristem results in the so-called diplontic selection. This competition between the mutated cell and the surrounding non-mutated cells is often lost by the mutated cell, causing a low frequency of mutated plants and a narrow mutation spectrum. When a mutated cell survives, chimeras are automatically formed because most apices consist of a number of fairly independent groups of cell layers.Such an undesirable situation can be improved by growing complete plants from only one cell, resulting in a high frequency of solid, non-chimera1 mutants and a wide mutation spectrum.Many plant species can be stimulated to form adventitious buds on isolated leaves and in a number of cases it has been reported that only one cell was involved.From study of literature a list has been prepared in which the various plant species which can be stimulated to form adventitious buds on isolated leaves are grouped, according to the location type of the "new-formations" and giving the bibliographical references from which the data have been taken. IhTRODUCTION A mutation is a unicellular event. In multicellular organisms, after irradiation of a multicellular apex, such mutated cell is exposed to the so-called diplontic selection, that is the competition between the mutated cell and the surrounding non-mutated ones. The mutated cell develops into a group of cells and finally into a cell layer, provided that neither the genetical change nor the physiological damage cause a decreased division rate and a limited outgrowth. The final result of the diplontic selection is a low number of mutated plants and a restricted mutation spectrum.Moreover, since most apices consist of a number of fairly independent groups of cell layers, mutation induction automatically leads to the formation of mericlinal and later to periclinal chimeras. In seed-propagated plants the chimera1 condition is discontinued after selfing or crossing; in vegetatively propagated plants the chimera1 condition is preserved as the so-called budsports, which occur abundantly in plants like Chrysanthemum, roses, carnation, potato and fruit trees.Early application of selection pressure in mericlinal shoots or plants is useless when such not immediately visible characters as yield, quality or resistance are involved. 321

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