||Chimerism is the concurrence of genotypically different tissues in one individual and usually results from a mutation early in the development of that individual. With the possibility to induce mutations came the problem of chimerism which gives heterogeneous plants and, allegedly, loss of mutations by diplontic selection, i.e. selection between genetically different tissues within an individual where the mutated tissue is assumed to be at a disadvantage.Literature on anatomy, morphology and development, and on the. mutation process gives the explanation for various aspects of chimerism: The appearance of chimerism depends on the constitution of the material treated and on the action of the mutagen used. Periclinal and sectorial chimerism are related to the structure of the apex i.e. the existence of independent cell layers and of a few central (apical) initial cells. The development of chimerism after a mutagenic treatment is determined by the differentiation already present in the material treated (usually seed), in which the destiny of most of the cells is already fixed.The presented experiments (on arabidopsis and sunflower) indicate that the mutagenic action of the mutagens used (EMS and colchicine applied separately or simultaneously) was mainly confined to the duration of the treatment. In addition the mutagens influenced development, stimulating it at first and later (especially colchicine) retarding it, which has consequences for sensitivity with combined treatments, and which may affect chimerism. Differences in average distribution of mutations (from seed versus seedling treatment, "first" versus "second" mutation) were ascribed to differential mutagen (EMS) sensitivity of cells destined to form the various parts of the plant (main inflorescence, side shoots). Chimerism in the sporogenic tissue (pollen and M 2 were scored) occurred as sectors (usually 2 in the inflorescence and up to 4 in the stem of arabidopsis) which often seemed twisted probably due to the (twisted) growth of the plants. Generally chimerism was lost, apparently at random with regard to the observed mutations (polypIoidy and M 2 chlorophyll, mutants).Although diplontic selection is often mentioned in the literature, most of the cases reported can be explained equally well, and often better, as aspects of normal development or as the result of differential (mutagen) sensitivity. Environmental conditions influence the development of the plant and also the (observed) mutation rate, but this influence may be indirect i.e. through the effect on chimerism. Chimerism depends on the stability of the apical initial cells which will be related to the stability of the apex and therefore to plant development and thus indirectly to environmental conditions.Plants from experiments with various growth conditions (greenhouse vs field, daylength) differed in development and vigour, and simultaneously, in degree and persistence of chimerism, vigorous plants showing more chimerism. Treatments which retard apical development at an early stage (etiolation, vernalization) decreased chimerism. In both cases effects on the stability of the apex were probably responsible.This loss of chimerism, at random with regard to the genotype, may be called diplontic drift. It depends on conditions affecting plant growth and may offer an opportunity to manipulate chimerism. Chimerism decreases the chance to detect a mutation but increases the number of (different) mutations that can be obtained from one plant, so that the degree of chimerism desired, may vary.