To imbibe and germinate urediniospores require free water or near saturated atmospheres in which both adsorption of water is maximal and capillary condensation pads form. The rate of spore germination is influenced by the amount of free water available. During imbibition, spores swell and capillary pads form beneath spores in contact with the substratum. These pads greatly increase the area of contact with the substratum. Pad area fluctuates and may depend on the balance between the rate of capillary condensation and the rate of uptake of fluid by spores. When spores, on leaves or cellulose acetate, are misted with water they imbibe rapidly and germinate. Adhesion pads which form under fully imbibed, germinating spores are morphologically similar to capillary pads but they differ in composition. On synthetic surfaces, during imbibition of spores and under microscopic glass beads, pads are composed principally of volatile capillary condensation and are removed when samples are freeze-dried. On host surfaces, when spores are fully imbibed and germinating, pads contain non-volatile soluble materials derived from both pathogen and host. These substances are not removed by extended freeze-drying.