FLORAL longevity, the length of time a flower remains open and functional, varies among plant species. Flowers of some species live less than one day (morning glory), whereas others live for several weeks (orchids)(1-3). By viewing floral longevity as a resource allocation stratey(2,4), we now incorporate the study of its evolution into the well developed theoretical framework provided by evolutionarily stable strategy models that address variation in life history(5,6). Flowers must remain open to contribute to plant fitness through ovule fertilization and pollen dissemination, when they require resources for respiratory maintenance and pollinator attraction. Accordingly, floral senescence should occur when the expected fitness gain per unit of floral maintenance investment diminishes to the point,where it becomes more profitable to construct a new flower than to maintain an existing one. Our experimental evidence supports floral longevity as an adaptation that balances rates of pollen receipt and removal against thr cost of floral maintenance.