Abstract:Sac-shaped wall structures were isolated from post-anthesis ovules of Antirrhinum majus treated with acetic anhydride-H2SO4 (9:1), 50% cold chromic acid, 80% H2SO4–30% H2O2 (5:1) or cellulase-pectinase solution. This means that the wall sac is acetolysis-resistant as well as enzyme-iesistant and has some resistance to strong oxidizers. Observations on paraffin and ultrathin sections confirmed that the isolated wall sac was just the inner wall of integumentary tapetum, which enclosed the embryo sac from all side but leaves an aperture at each pole. This wall showed positive reaction to Sudan IV staining and auramine O fluorescence staining but negatively reacted to phloroglucinol, indicating that it was likely to be composed of highly stable lipid substances, preferably cutin or sporopollenin. Based on all these results, it is supposed that the wall sac functioned as a dam for preventing damage of young embryo and endosperm from the hydrolytic enzymes secreted by integumentary tapetum and also for chanalizing nutrient flow through the chalazal and micropylar apertures into embryo sac.