R. Freund, M. Oswald, A. Paun:
"Optimal Results for the Computational Completeness of Gemmating (Tissue) P Systems";
International Journal of Foundations of Computer Science, 16 (2005), 5; 929 - 942.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936282