Abstract
Sporelings of the marine red alga Plumaria elegans, grown under continuous illumination from a fluorescent tube, show a maximum rate of cell production at a light energy flux of 10 ergs/sec/mm2 in the range 380—720 mμ. This amount of light energy is low, but compares closely with that illuminating Plumaria in its natural habitat at Wembury during the period of the year when the plant is producing spores.