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Caffeine and theophylline were found to increase the period length of the phototactic rhythm of Chlamydomonas reinhardii in a dose-related manner. Treatment with caffeine resulted in average period increase of 11.2% (3 mM), 16.4% (5 mM), and 29% (10 mM). Theophylline caused smaller period increases than did caffeine: an average period lengthening of 4.2% (3 mM) and 6.1 % (5 mM).
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Goodenough et al. (1980) studied this question.
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