ings. This coincides with the findings of several authors concerning TE development in in vitro culture (recently reviewed in [13]). During the conversion from naked mesophyll protoplasts to TE there occurred intermediate forms which showed characteristics of both mesophyll cell and TE. This was particulary obvious when intact green chloroplasts and a nearly complete secondary wall pattern could be observed simultaneously in the same cell (Fig. 1 d). The predominant part of TE differentiated without preceding cell division. TE which were formed after a division were rare as were cells with only partially thickened cell walls. By the way our experiences with both Zinnia cells and protoplasts showed that the protoplast system can be much better standardized because for protoplast isolation sterile cultured plants can be used. The experiments are continued with the aim (among others) to increase the yield of TE. If this should be the case, the system offers the chance for examining a fundamental phenomenon of cytodifferentiation under aspects of somatic genetics. It will present the advantage to influence the process of differentiation by introduction of foreign genetic material via incorporation or fusion experiments.