The part of the host plasmalemma invaginated around the haustorium of Pseudoperonospora humuli was examined after staining with (a) Uranyl acetate followed by lead citrate and (b) a mixture of phosphotungstic acid and chromic acid. It was also examined by using a tilting and rotating goniometer stage.
After staining with uranyl acetate and lead citrate the membrane could only be detected on about 45 % of the periphery of the haustorium transected in a section and only the outer dark component of the triplet unit membrane structure could be detected. The use of phosphotungstic acid-chromic acid enabled the whole of the triplet membrane structure to be seen but did not enable the membrane to be detected over a greater percentage of the haustorium periphery than with the first procedure.
The use of the goniometer stage resulted in the membrane being seen around 95 % of the periphery of the haustorium.