ABSTRACTUredospore germlings ofUromyces appendiculatuswere induced by K+, Ca++, or sucrose in order to form appressoria chemotropically to determine more clearly the mode of differentiation. Differentiation occurred only when the germlings grew aerially, away from the solutions or agar media containing KCl, CaCl2, or sucrose. Germlings submerged in these solutions differentiated only when a topographically inductive surface was provided. The microtubule cytoskeleton was symmetrically arranged in germlings grown aerially and asymmetrically in germlings grown on a firm substrate.