Gametophytes and homozygous sporophytes containing this mutation accumulate massive amounts of starch in their plastids when grown under constant light conditions. This can be easily observed by viewing the plastids under a compound microscope (20+X); staining with I2KI can enhance observations. The massive starch grains give the plastids a very ‘lumpy’ appearance. The accumulation of starch suggests that under constant light conditions the cells are unable to effectively use photosynthate and, as a result, growth is severely impaired. This is a conditional mutation in that near normal growth and depletion of the abnormal starch accumulation occur when gametophytes are grown under day-night conditions. Sporophytes grow well under normal greenhouse day-night conditions. X-ray induced, recessive in sporophytes.