A typical middle school classroom is used by students (and a teacher) every weekday morning, and is not used otherwise. The windows are kept closed during the study. There is a small indoor garden in the classroom, with plants. A CO2 sensor was placed in the classroom, and CO2 levels in the classroom were measured over an 8-day period. The results of the sensor measurements produced a distinctive pattern of CO2 levels over time, seen in the graph below, and the class is wondering what could cause this pattern.