In multicellular plants, development starts with an asymmetric division of the zygote into two differentiated cells. The nature and distribution of fate-determining factors operating during embryogenesis remain largely obscure. Laser microsurgery was used here to dissect two-celled embryos of the alga Fucus spiralis. Removal of protoplasts from the cell wail induced dedifferentiation. However, isolated cells within the walls followed their restricted fate. Moreover, contact of one cell type with the isolated cell wall of the other cell type caused its fate to be switched. The cell wall thus appears to maintain the differentiated state and to direct cell fate in plant development.