Scientists have found that it is the amount of darkness, not the amount of sunlight, that determines the flowering of plants. This relates to photoperiodism, the physiological reactions of an organism to the length of day or night. In many angiosperms photoperiodism enables plants to sense seasonal changes and accordingly start to flower. Long-day plants specifically flower when day length exceeds a certain threshold, called their “critical photoperiod”. This threshold is usually around 12 hours. These plants usually flower in late spring or early summer, because the days start to get longer and the nights start to get shorter. One example of a long-day plant is the potato, shown in the image above.