Embryogenic cell lines with lasting embryogenic potential can be obtained from somatic embryos induced directly from zygotic embryos of Arabidopsis thaliana, ecotype Columbia. The response to a critical concentration of auxin, which seems to be the all-important factor in the generation of embryogenic cell lines, is exhibited by somatic embryos but not by zygotic ones. The basis for this differential response remains obscure and will be discussed in relation to other systems.