Transpiration, leaf area and a source-sink tradeoff

A school exercise supplies mass-loss observations for two matched potted plants under different air humidities. Soil surfaces are covered so that the intended measured loss is mainly through leaves, and both plants have leaf area 100 square centimeters. The simplified model neglects growth-related mass change and changes in internal water storage over the one-hour observation. Under humid air the recorded loss is 2 grams; under drier air it is 6 grams. Light, temperature and other stated conditions are treated as matched. A student compares the raw losses without mentioning leaf area, and another suggests that closing all stomata would improve every aspect of plant performance. The class must normalize the observations, explain a plausible water-vapor gradient and identify why water conservation can also constrain carbon acquisition.