The culture of bent‐cotyledon‐stage zygotic embryos of Arabidopsis thaliana in a medium containing the synthetic auxin 2,4‐dichlorophenoxyacetic acid (2,4‐d) for 10 d induces the formation of early‐stage somatic embryos on the callus formed on the cotyledons. Transferring the callus to an auxin‐free basal medium leads subsequently to the formation of mature‐stage somatic embryos. The timing of formation of mature‐stage somatic embryos in Arabidopsis by 2,4‐d action was studied by monitoring the morphogenetic changes in zygotic embryos transferred to the basal medium and grown in light and/or dark conditions after different periods of treatment in a medium containing 2,4‐d. When embryos subjected to increasing periods of 2,4‐d action are grown in the basal medium, they form, in transitional order, normal seedlings; abnormal seedlings, consisting of a short stem with ovate, spoon‐shaped, and bifid or lobed leaves and adventitious leafy shoots; a callus of cotyledonary origin, bearing a mixture of leaves and tubular somatic embryos with fused, bifid, or lobed cotyledons; and, finally, a callus with mature‐stage tubular somatic embryos with fused, bifid, or lobed cotyledons. During growth in the basal medium in light, cotyledons of somatic embryos formed on zygotic embryos pretreated for suboptimal periods with 2,4‐d acquire the morphological and anatomical features of leaves by the formation of trichomes and the typical venation pattern, consisting of a midvein and secondary veins separating the cotyledon into two or more chambers. Somatic embryos in which cotyledons are transformed into leaves phenocopy zygotic embryos of the leafy cotyledon1 mutant of Arabidopsis. The transformation into leaves of cotyledons of somatic embryos regenerated on zygotic embryos subjected to suboptimal periods of 2,4‐d action indicates that a defective synthesis or maintenance of the local concentration of auxin during embryogenesis might account for this mutation.