Gymnosporangium sabinae , mostly on Pyrus communis: a. Spermatogonia in orange leaf spots on the upper leaf surface; at this stage, no swellings of the leaf tissue and no aecia are visible yet; b. Aecia on gall-like swellings on the lower leaf surface (on Pyrus pyraster); c. Old aecia; d. Two aecia at higher magnification; note that the peridium is regularly ruptured into fine fibres but its apex remains intact, also after spore release; (b by Julia Kruse; d by Walter Obermayer).