During peak thermogenesis of anthesis, high rates of respiration by the sterile male florets on the spadix ofPhilodendron selloum significantly reduce the oxygen tension (PO 2) and raise CO2 tension between the florets. Nevertheless, respiration is not limited by the availability of O2 under natural conditions. At experimental PO 2 below about 17 kPa, however, florets begin to show decreased O2 consumption. A respiratory exchange ratio of 0.83 indicates that the major energy source is not starch, but is probably lipid.