光周期的暗期长度是决定植物成花的决定因素。
The darkness length in photoperiod is the crucial factor determining the formation of plant flowers.
但是，两组光周期被打乱的老龄雄鼠的死亡率则高多了。
But the number of deaths in the two groups whose day-and-night cycles had been tampered with was far higher.
人们认为这种梯度变异可能是降水量，温度也许还有光周期等地理趋势造成的结果。
The cline is considered the probable result of geographic trends in precipitation, temperature, and perhaps also photoperiod.
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