Part 1 of Essay
Plants and animals in the temperate zones respond in different ways to the amount of daylight in 24-hour periods. This response to day length is called photo periodism. It controls many activities, among them is the flowering of plants. The ability to respond to day length is linked to an inner, light-sensitive circadian ....

Part 2 of Essay
.... of light, plants can measure day length to determine the season and the time spans within a season. Florists can often manipulate greenhouse plants into producing blossoms out of season by exposing them to periods of artificial light. Some scientists are not certain that the biological clock of any organism is completely endogenous. They think that even under the most constant of laboratory conditions living things are aware of the Earth's rotation and that this has an effect on the wheelof their clocks. ....