AbstractElectron micrographs ofPisum sativumL. hypocotyl tips treated to localize adenyl cyclase revealed discrete deposits on the internal membranes of cytoplasmic vacuoles which correspond to previously localized enzymes described as acid phosphatases. It remains to be determined whether the specificity of the substrate, adenylyl‐imidodiphosphate, used in the present study is such as to exclude all phosphatase activity other than adenyl cyclase. The acid phosphatase localized in earlier studies by other investigators may be an adenyl cyclase. In the differentiated cells of the root cap, lead precipitate was localized in distinct areas bound to the smooth endoplasmic reticulu