the Oidium anamorph of Erysiphe graminis, whitish chains of conidia (the 'powdery mildew') cover the host leaves. Each chain consists of a graded series of gradually maturing conidia, the oldest at the tip, the youngest barely differentiated from the hyphal cell just below it. New material is added at the base of the chain in a form of intercalary growth, arising from a sometimes swollen mother cell which appears to be a highly modified phialide.