The conchocelis-phase of Porphyra tenera Kjellman has been grown in in vitro culture, and subjected to a variety of daylength treatments.
Details of the culture apparatus designed for these treatments are given, and quantitative techniques, which have been devised for estimating the growth and reproduction of the alga, are described and discussed.
Growth was found to be directly proportional to daylength within the range 8–16 h, but abundant sporangia were only formed in daylengths of 10 h or less. The interruption of a 16 h dark period by 1 h of light completely inhibited the production of sporangia, but exposure to 8 short-day cycles, followed by long days, induced the formation of as many sporangia as were found after 25 short days. These results are discussed with reference to the photoperiodic responses of flowering plants.