O2‐consumption of sprouting potato plugs in constant conditions was monitored hourly over ten years. The data for each month was reorganized to obtain the mean rate for each hour of the lunar day. All the data revealed a very low amplitude residual lunar‐day pattern, range less than 1%, with a lesser minimum at upper transit of the moon and major maximum at lower transit. Hourly variances disclosed a significantly greater range from upper transit to lower than for the rest of the day. To discover possible inverting tendencies for the lunar cycles, the mean monthly days were divided into quarters with each quarter expressed as its deviation from the daily mean. Greater negative correlation than random (—0.333) would indicate an inverting semidiurnal cycle if found between adjacent quarters, and a diurnal if between alternate ones. A diurnal cycle was demonstrated (P<.001) using all data when the quarters included centering on upper transit but not when the quarter terminated at this time. Using the upper‐transit‐centered quarterly series an annual modulation was found, with a highly significant diurnal cycle (P<.000001) for the months September‐February and only randomness for March‐August. The results during fall and winter indicate a family of inverting diurnal lunar‐day cycles with maxima at upper transit, lower transit, moonrise, or moonset. Neither diurnal nor semidiurnal lunar cycles occurred the remainder of the year.