A metamorphosis from rosette to inflorescence in many annuals shifts photosynthetic tissue from a two-dimensional array in the soil boundary layer during cool months to a three-dimensional structure in the troposphere as spring progresses. We propose that this shift allows escape from both self-shading and an increasingly stressful boundary layer microclimate, permitting continued increases in growth. As a first step in exploring this hypothesis, we compared the lifetime C gain, water loss, and instantaneous water use efficiency (WUE) of five Arabidopsis thaliana genotypes by measuring gas exchange across the life cycle. On average, the inflorescence contributed 55% (± 5% SE) of lifetime C gain, but only 25% of lifetime water loss. Mean inflorescence WUE was nearly fourfold that of the rosette. The inflorescence continued to fix C after rosette senescence. The percentage inflorescence: total C gain varied among genotypes, from 36% to 93%. Genotypes differed in WUE for both structures. We suggest that local climates may have selected for divergence in these traits. For many annuals and winter annuals, understanding C and water budgets and their evolution must include measures of both rosette and inflorescence gas exchange.