To protect books against fungi, it is necessary to understand the physiology of these organisms and the moisture-holding properties of different papers under varying conditions of storage. Spore germination is possible in various types of paper at between 45 to 90% RH and 14° to 22°C. A system of rectangular coordinates illustrates three relative humidity/temperature regions where the fungal spores 1) cannot germinate, 2) germinate on certain types of paper, 3) germinate on all types of paper.