Internal calibration of electron micrographs with an Orchid virus.
The length of particles of orchid (Odontoglossum) ring spot virus [cf. 30: 610] has been established as 300 mµ [31: 434] and they are shown to be a convenient standard for calibration of electron micrographs. Outside the orchid family 4 Chenopodium spp. were found susceptible to the virus and convenient hosts for it, developing local necrotic lesions 5-8 days after inoculation and facilitating rapid identification. Using this method accurate particle lengths were obtained for potato virus X, watermelon mosaic, and bean yellow mosaic viruses.