Experiments with photoperiods ranging from 2 to 24 h confirm that 8 h light per day is optimal for Lunularia: there is no growth in the dark or in continuous light, which causes the rapid onset of dormancy. Short-day cycles intercalated among a series of continuous light cycles promote growth; in cycles longer than 24 h very long dark periods are detrimental. With very short photoperiods (5 min) red light promotes growth more effectively than white light at higher intensity; far-red acts as dark. The growth effects of red and far-red light breaks (3 min) depended on the time of application; red light inhibited in the middle but promoted at the beginning of the 16-h dark period of a short day; far-red light had the opposite effect; in each case red and far-red effects were reversible by the other wavelength. Blue light gave the same response as red including the reversibility of far-red effects and vice versa. Surprisingly, significant effects of 5 min red, blue, and far-red irradiation were also found in the middle of the main high-intensity white-light period, red and blue promoting growth, far-red reducing it; again there was ready reversibility of the effects. Growth promoters of higher plants are generally inhibitory to Lunularia or have little effect; among growth retardants TIBA, Phosphon D, and CCC gave a slight promotion of growth. EDTA promoted growth (cell numbers) very significantly while 8-hydroxyquinoline was initially inhibitory, but had a marked latent promoting effect when subsequently washed from the thalli.