Leaves have three striking morphological features. Firstly, they are much longer and wider than thick, secondly they are produced laterally from the stem axis and thirdly, they exhibit dorsoventrality. To understand better the mechanisms that lead to dorsoventrality in the lateral organs of plants the morphology of mutants of the phantastica (phan) locus of Antirrhinum majus were characterised. The leaves, bracts, and petal lobes of phan mutants showed varying degrees of reduction in dorsal tissues, indicating that phan is required for establishing dorsal cell identity. Each phan mutant produces a similar variety of different leaf morphologies, but has a characteristic and relatively constant floral phenotype. In several different forms of phan mutant leaves and petal lobes, novel boundaries between dorsal and ventral cell types form ectopic axes of growth, suggesting that phan-dependent dorsal identity is required for lateral growth of the wild-type leaf and petal lobe. Comparisons between the development of wild-type and mutant petals or leaves reveal that phan acts early in development of these lateral organs. To allow molecular characterisation of the phan gene an attempt to isolate the locus was undertaken. Stable phan mutant lines were crossed to transposon active lines and 18,000 F1 progeny screened for a new phan mutant allele. One new mutant allele, phan-522, was identified and this showed the genetic instability characteristic of transposon-induced mutations. A new insertion of the transposon Tam 4 was identified that appeared to be responsible the phan-552 mutation. It was present in the phan-552, but not in eight wild-type siblings and had been lost in 31 Phan⁺ revertants in nine families produced by back-crossing the phan-552 mutant to its stable mutant parent. This copy of Tam 4 was isolated and DNA flanking the transposon shown to include part of the phan transcription unit.