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Induction of male strobili in Arizona cypress (Cupressus arizonica Greene) by exogenous gibberellin A3 (GA3) appears to have an absolute requirement for light, a quantitative requirement for long day (LD), and as shown by interruption of the dark periods, is in part under photoperiodic control. Under LD conditions the total number of new meristems produced is 10-fold greater than under short day (SD) even though this species exhibits indeterminate growth and does not set dormant buds under SD. Hence the total number of strobili resulting from GA3 application is higher under LD than under SD. However, strobili produced by GA3, when expressed as a percentage of the total number of meristems, is the same under both LD and SD, provided the GA3 concentration under SD is increased, and a longer period of time is allowed under SD. The factor(s) responsible for the enhanced meristem production under LD decrease(s) to an apparent steady state within the first 10 days after transfer of the plant to SD.