noun Biology.
the response, as affecting growth or reproduction, of an organism to the length of exposure to light in a 24-hour period.
Also called photoperiodicity [foh-toh-peer-ee-uh-dis-i-tee] .

1915–20; photoperiod + -ism Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
photoperiodism (ˌfəʊtəʊˈpɪərɪəˌdɪzəm)
the response of plants and animals by behaviour, growth, etc, to photoperiods

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

photoperiodism pho·to·pe·ri·od·ism (fō'tō-pĭr'ē-ə-dĭz'əm) or pho·to·pe·ri·o·dic·i·ty (-pĭr'ē-ə-dĭs'ĭ-tē)
The response of an organism to changes in its photoperiod, especially as indicated by vital processes.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
photoperiodism (fō'tō-pîr'ē-ə-dĭz'əm) also photoperiodicity   (fō'tō-pîr'ē-ə-dĭz'əm)  Pronunciation Key 
The response of an organism to changes in its photoperiod, especially as indicated by vital processes. For example, many plants exhibit photoperiodism by flowering only after being exposed to a set amount of daylight, as by requiring either a long or short day to flower. Plant growth, seed germination, and fruiting are also affected by day length. Photoperiodic responses in plants are regulated by special pigments known as phytochromes. In animals, migration, mating, amount of sleep, and other behaviors are also photoperiodic. In many animals, photoperiodism is regulated by the hormone melatonin.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Discovered that flowering in many plants is controlled by changes in day-length, a concept called photoperiodism.
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